Retry bw_usage_update() on innodb Deadlock

Adds a new decorator _retry_on_deadlock() to sqlalchemy api.  This patch
makes bw_usage_update() use it.

Fixes bug 1129622

Change-Id: I0293c62d2dd5ac036445bc639cabbd05ba016e83
Merge "Retry bw_usage_update() on innodb Deadlock"