rdbms: Support secondary autocommit connections in LoadBalancer

This is useful for things like SqlBagOStuff and JobQueueDB
being able to write in auto-commit mode while the main
transaction round still goes on.

SqlBagOStuff already had this sort of logic.

JobQueueDB now also takes this approach so it does not have to
defer insertion except for sqlite. This makes callers more likely
to know when insertion failed or not.

Make sure LoadBalancer sets "master"/"replica" LB info itself;
this fixes a bug found in the new tests.

Bug: T140338
Bug: T42451
Change-Id: I4199a9598d7afbf976a6efa8ed84b85b56da02bd