nexus: use all CockroachDB hosts from DNS to create DB connection URL.

[luqmana]

First pass at #3763 for crdb.

Even though we did query internal DNS, we were previously using only a single host as part of connecting to crdb from Nexus. And since the internal DNS server always returns records in the same order, that meant every Nexus instance was always using the same CockroachDB instance even now that we've been provisioning multiple. This also meant if that CRDB instance went down we'd be hosed (as seen in #3763).

To help with that, this PR changes Nexus to use all the CRDB hosts reported via Internal DNS when creating the connection URL. There are some comments in the code, but this still not quite as robust as we could be, but short of something cueball-like it's still an improvement.

To test I disabled the initial crdb nexus connected to and it was able to recover by connecting to the next crdb instance and continue serving requests. From the log we can see a successful query, connection errors once i disabled fd00:1122:3344:101::5, and then a successful query with connection reestablished to next crdb instance (fd00:1122:3344:101::3):

23:43:24.729Z DEBG 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): authorize result
    action = Query
    actor = Some(Actor::UserBuiltin { user_builtin_id: 001de000-05e4-4000-8000-000000000003, .. })
    resource = Database
    result = Ok(())
23:43:24.729Z ERRO 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): database connection error
    database_url = postgresql://root@[fd00:1122:3344:101::5]:32221,[fd00:1122:3344:101::3]:32221,[fd00:1122:3344:101::6]:32221,[fd00:1122:3344:101::4]:32221,[fd00:1122:3344:101::7]:32221/omicron?sslmode=d
isable
    error_message = Connection error: server is shutting down
23:43:24.729Z ERRO 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): database connection error
    database_url = postgresql://root@[fd00:1122:3344:101::5]:32221,[fd00:1122:3344:101::3]:32221,[fd00:1122:3344:101::6]:32221,[fd00:1122:3344:101::4]:32221,[fd00:1122:3344:101::7]:32221/omicron?sslmode=d
isable
    error_message = Connection error: server is shutting down
23:43:24.729Z ERRO 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): database connection error
    database_url = postgresql://root@[fd00:1122:3344:101::5]:32221,[fd00:1122:3344:101::3]:32221,[fd00:1122:3344:101::6]:32221,[fd00:1122:3344:101::4]:32221,[fd00:1122:3344:101::7]:32221/omicron?sslmode=d
isable
    error_message = Connection error: server is shutting down
23:43:24.729Z ERRO 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): database connection error
    database_url = postgresql://root@[fd00:1122:3344:101::5]:32221,[fd00:1122:3344:101::3]:32221,[fd00:1122:3344:101::6]:32221,[fd00:1122:3344:101::4]:32221,[fd00:1122:3344:101::7]:32221/omicron?sslmode=d
isable
    error_message = Connection error: server is shutting down
23:43:24.729Z ERRO 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): database connection error
    database_url = postgresql://root@[fd00:1122:3344:101::5]:32221,[fd00:1122:3344:101::3]:32221,[fd00:1122:3344:101::6]:32221,[fd00:1122:3344:101::4]:32221,[fd00:1122:3344:101::7]:32221/omicron?sslmode=d
isable
    error_message = Connection error: server is shutting down
23:43:24.729Z ERRO 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): database connection error
    database_url = postgresql://root@[fd00:1122:3344:101::5]:32221,[fd00:1122:3344:101::3]:32221,[fd00:1122:3344:101::6]:32221,[fd00:1122:3344:101::4]:32221,[fd00:1122:3344:101::7]:32221/omicron?sslmode=d
isable
    error_message = Connection error: server is shutting down
23:43:24.730Z ERRO 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): database connection error
    database_url = postgresql://root@[fd00:1122:3344:101::5]:32221,[fd00:1122:3344:101::3]:32221,[fd00:1122:3344:101::6]:32221,[fd00:1122:3344:101::4]:32221,[fd00:1122:3344:101::7]:32221/omicron?sslmode=d
isable
    error_message = Connection error: server is shutting down
23:43:24.730Z ERRO 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): database connection error
    database_url = postgresql://root@[fd00:1122:3344:101::5]:32221,[fd00:1122:3344:101::3]:32221,[fd00:1122:3344:101::6]:32221,[fd00:1122:3344:101::4]:32221,[fd00:1122:3344:101::7]:32221/omicron?sslmode=d
isable
    error_message = Connection error: server is shutting down
23:43:30.803Z DEBG 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): roles
    roles = RoleSet { roles: {} }
23:43:30.804Z DEBG 7be03b0d-48bf-4f43-a11e-7303236a3c5e (ServerContext): authorize result
    action = Query
    actor = Some(Actor::UserBuiltin { user_builtin_id: 001de000-05e4-4000-8000-000000000003, .. })
    resource = Database
    result = Ok(())