Fixes: #4663 : Move the explicit in `DefaultDialect` initialization to

Fixes: #4663 : Move the explicit in `DefaultDialect` initialization to
the pool's `first_connect` handler. This ensures that all dialects will
always roll back after completing their call to `initialize(conn)`.
This has the effect of ensuring that the first connection to a postgresql
database is in the same state as all subsequent connections, i.e. not with
an implicit transaction open from the feature detection queries.