This commit fixes an issue with the series file compaction process
where tombstones are lost after compaction and series existence
checks are not correct. This commit also fixes some smaller flushing
issues within the series file that mainly related to testing.
The series file compaction previously did not snapshot the max
offset before compacting and would keep compacting until it reached
the end of segment file. This caused more entries than expected into
the RHH map and this map gets exponentially slower as it gets close
to full.
Ben Johnson