async flush, bugfixes, event variant rename

Breaking Changes

• Event::Set has been renamed to Event::Insert and
  Event::Del has been renamed to Event::Remove. These
  names better align with the methods of BTreeMap from
  the standard library.

Bug Fixes

• A deadlock was possible in very high write volume
  situations when the segment accountant lock was
  taken by all IO threads while a task was blocked
  trying to submit a file truncation request to the
  threadpool while holding the segment accountant lock.

New Features

• flush_async has been added to perform time-intensive
  flushing in an asynchronous manner, returning a Future.