TEPHRA-43 TransactionVisibilityFilter can prematurely drop delete markers

This fixes all versions of TransactionVisibilityFilter so that we only drop "delete" tombstones (Puts with empty values) when all store files are being read at the same time.  Makes the following changes:
* adds a flag to TransactionVisibilityFilter, set when doing a user scan or major compaction
* adds a test case that verifies "delete" tombstones are kept after a memstore flush
We could encapsulate the logic in the TransactionVisibilityFilter constructor.  I guess the question is: is the boolean argument or ScanType enum easier to understand?