oxenstored: perform a 3-way merge of the quota after a transaction

oxenstored: perform a 3-way merge of the quota after a transaction

At a beginning of a transaction, the quotas from the global store
are duplicated and modified by the transaction. If during the
transaction, an action associated to no transaction is concurrently
executed, the quotas of the global store are updated, and then the
updates are lost when the transaction merges.

We fix this problem by keeping another copy of the quota at the
beginning of the transaction, and performing a 3-way merge between
the quotas from the transaction and the "original" copy of the quota
onto the quota of the global store.

Signed-off-by: Jerome Maloberti <jerome.maloberti@citrix.com>
Signed-off-by: Euan Harris <euan.harris@citrix.com>
Acked-by: David Scott <dave.scott@citrix.com>