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Auto recovery from out of space errors

Summary:
This commit implements automatic recovery from a Status::NoSpace() error
during background operations such as write callback, flush and
compaction. The broad design is as follows -
1. Compaction errors are treated as soft errors and don't put the
database in read-only mode. A compaction is delayed until enough free
disk space is available to accomodate the compaction outputs, which is
estimated based on the input size. This means that users can continue to
write, and we rely on the WriteController to delay or stop writes if the
compaction debt becomes too high due to persistent low disk space
condition
2. Errors during write callback and flush are treated as hard errors,
i.e the database is put in read-only mode and goes back to read-write
only fater certain recovery actions are taken.
3. Both types of recovery rely on the SstFileManagerImpl to poll for
sufficient disk space. We assume that there is a 1-1 mapping between an
SFM and the underlying OS storage container. For cases where multiple
DBs are hosted on a single storage container, the user is expected to
allocate a single SFM instance and use the same one for all the DBs. If
no SFM is specified by the user, DBImpl::Open() will allocate one, but
this will be one per DB and each DB will recover independently. The
recovery implemented by SFM is as follows -
  a) On the first occurance of an out of space error during compaction,
subsequent
  compactions will be delayed until the disk free space check indicates
  enough available space. The required space is computed as the sum of
  input sizes.
  b) The free space check requirement will be removed once the amount of
  free space is greater than the size reserved by in progress
  compactions when the first error occured
  c) If the out of space error is a hard error, a background thread in
  SFM will poll for sufficient headroom before triggering the recovery
  of the database and putting it in write-only mode. The headroom is
  calculated as the sum of the write_buffer_size of all the DB instances
  associated with the SFM
4. EventListener callbacks will be called at the start and completion of
automatic recovery. Users can disable the auto recov ery in the start
callback, and later initiate it manually by calling DB::Resume()

Todo:
1. More extensive testing
2. Add disk full condition to db_stress (follow-on PR)

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Summary:
1. Fix corner cases where compaction state is left inconsistent
  a) Compaction triggered by recovery
  b) Compaction failure after writes are stopped
  (level0_stop_writes_trigger) - add it back to compaction queue
2. Prevent indefinite wait in DBImpl::DelayWrite()
3. Fix required free space calculation in SstFileManagerImpl

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Tested using db_bench on a loopback filesystem and periodically filling
up the fs

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Summary:
1. Rebase and resolve conflicts
2. Work around GetFreeSpace conflict in Windows build
3. Misc. Travis compilation errors

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.travis.yml with a specific config and coredump/gdb enabled

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Summary:
1. Add tests for multiple column family and multiple DB cases
2. Change SstFileManager::OnAddFile signature
3. Better isolate DB instances in the multiple instance case by not
impacting one instance if another has run into error, in order to
preserve existing behavior

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The main conflict is with the commit 7daae51, which refactors flush
request processing.

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Temporarily disbling these tests in Travis as they are only failing in
Travis and not reproducible elsewhere.

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