Allowing partial commits of Segments being batch compacted.

[simerplaha]

Currently as Segments are batch compacted from one Level to another a failure to commit any one Segment will revert the entire compaction. The compaction is retried again based on the throttle configuration.

These failure can occur if a Segment is closed(maxSegmentsOpen) while its being read.

To avoid retries and to save IO partial commits of the batched Segments should be allowed - to write as many Segments as possible and alert the upper Level of the partial commit where only the failed Segments will be retried.

[simerplaha]

Fixing this issue will be a minor improvement to the compaction as these failures can be avoided entirely by giving maxSegmentsOpen a large number.

[simerplaha]

Commits can now be spread to multiple lower levels to be considered full committed. Issue #11 basically resolves this.