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
Nevermind, that doesn't work because the RegionCoprocessorHost does not get instantiated if RegionServerServices is null, which means we have to call the HRegion constructor.

We could add a test helper in the future once we have more to add to it, but I don't think it's worth it with only this method.