Also at seastar-dev: calle/commitlog_flush_v3
(And, yes, this time I _did_ update the remote!)
Refs #262
Commit of original series was done on stale version (v2) due to authors
inability to multitask and update git repos.
v3:
* Removed future<> return value from callbacks. I.e. flush callback is now
only fully syncronous over actual call