Update the DurabilityMonitor internal tracking and commit in a background thread

Currently the following functions are executed in front-end threads:

• DurabilityMonitor::addSyncWrite
• DurabilityMonitor::seqnoAckReceived

Both functions:

1. update the internal tracking
2. commit satisfied SyncWrites

Both operations a relatively slow for being executed in a front-end thread, so we expect a relevant (negative) impact on throughput.

The idea is to do just the strictly necessary in front-end thread (e.g., register that a Seqno-Ack has been received), and defer all the rest to a BG execution (e.g., update iterators and commit).