rdbms: Add IDatabase::cancelAtomic()

rdbms: Add IDatabase::cancelAtomic()

Atomic sections are currently useful if you want to wrap some SQL
statements in a transaction when you might be called from inside someone
else's transaction, and you expect the caller to roll back everything if
you fail.

But there are some cases where you want to allow the caller to recover
from errors, in which case you need to roll back just the atomic
section. Savepoints are supported by all our databases and can be used
for this purpose, so let's do so.

Bug: T188660
Change-Id: Iee548619df89fd7fbd581b01106b8b41d3df71cc