"Fixes #262
Handles CL disk size exceeding configured max size by calling flush handlers
for each dirty CF id / high replay_position mark. (Instead of uncontrolled
delete as previously).
* Increased default max disk size to 8GB. Same as Origin/scylla.yaml (so no
real change, but synced).
* Divide the max disk size by cpus (so sum of all shards == max)
* Abstract flush callbacks in CL
* Handler in DB that initiates memtable->sstable writes when called.
Note that the flush request is done "syncronously" in new_segment() (i.e.
when getting a new segment and crossing threshold). This is however more or
less congruent with Origin, which will do a request-sync in the corresponding
case.
Actual dealing with the request should at least in production code however be
done async, and in DB it is, i.e. we initiate sstable writes. Hopefully
they finish soon, and CL segments will be released (before next segment is
allocated).
If the flush request does _not_ eventually result in any CF:s becoming
clean and segments released we could potentially be issuing flushes
repeatedly, but never more often than on every new segment."
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