Unify _commit_puts for accounts and containers

The _commit_puts method is exactly the same in AccountBroker and
ContainerBroker, except for what it puts on the list pending records.
It seems to be easy to factor out without victimizing the code
clarity unduly.

We also factor common checking for LockTimeout: not so much for
any code savings, but to underscore that all these little segments
really are the same, so a reader does not need to prove uniformity
every time.

Change-Id: Idf9b553a3cf28de3c8803aaa47cf9a2be6817f00