Use statement_timestamp() instead of now()

[andrepiske]

Postgresql has a handful of functions to get the current timestamp, with varying exact meanings for each one.

So far we've been using the NOW() function which returns the timestamp of the beginning of a transaction block rather than the actual clock time. This PR changes it to use the STATEMENT_TIMESTAMP() function instead, which returns the timestamp of the beginning of a statement. Postgres documentation does a better job of explaining how exactly each of those functions work: https://www.postgresql.org/docs/current/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT

In rails, a common pattern is to do

ActiveRecord::Base.transaction do
  ...
end

and in this case, the timestamp returned by NOW() will be set by when ActiveRecord::Base.transaction is called and not when statements inside the block are issued.

There are ups and downs to using each one and no right answer as to which one to use. But we feel that with this change, timestamps will behave more like a wall clock, especially in long or slow transactions.

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Related tasks:

• INF-234