Implement atomic refs update, if possible by database

Inspired by the series[1], this implements the possibility to
have atomic ref transactions.
If the database supports atomic ref update capabilities, we'll
advertise these. If the client wishes to use this feature, either
all refs will be updated or none at all.

[1] http://thread.gmane.org/gmane.comp.version-control.git/259019/focus=259024

Change-Id: I7b5d19c21f3b5557e41b9bcb5d359a65ff1a493d
Signed-off-by: Stefan Beller <sbeller@google.com>
Merge "Implement atomic refs update, if possible by database"