- Disabled by default, enable with client_enable_writeback=true
- Even then only enabled in FIO when -direct is disabled and in QEMU when
block device cache is enabled in settings
- Can also be enabled in other clients like vitastor-cli using parameter
client_writeback_allowed=true, but not recommended
Previous implementation didn't respect write ordering and could lead
to corrupted data when restarting writes after an OSD outage
Also rework cluster_client queueing logic and add tests for it to verify the correct behaviour