[nexus] Remove SELECT FOR UPDATE, patch tests (#6729)

Building off of:

-
#6229 (comment)
- #6694
- https://github.com/oxidecomputer/sqldance

This PR acknowledges that `SELECT FOR UPDATE` is a performance
"optimization" in the situation where a blueprint's value is checked
before performing a subsequent operation, as we do with setting network
resources. It arguably could make performance worse in certain cases, as
it locks out concurrent read operations from accessing the database.

This PR removes the usage of `SELECT FOR UPDATE`, and significantly
overhauls the `test_ensure_external_networking_bails_on_bad_target` test
to account for the concurrent control that CockroachDB may be performing
with respect to operation re-ordering.

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Co-authored-by: John Gallagher <[email protected]>