oxenstored: support commit history tracking

oxenstored: support commit history tracking

Add ability to track xenstore tree operations -- either non-transactional
operations or committed transactions.

For now, the call to actually retain commits is commented out because history
can grow without bound.

For now, we call record_commit for all non-transactional operations. A
subsequent patch will make it retain only the ones with side-effects.

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>
Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
Reviewed-by: Christian Lindig <christian.lindig@citrix.com>