Add test and fix issue with concurrent drop and usage of legacy index.

Legacy index creation and deletion is not transactional, but its availability
was treated as such during the commit to store phase. If a particular index cannot
be found then it has been deliberately removed and thus any changes to it should
be directed to the bitbucket.
Merge pull request #3601 from martinfurmanski/2.2-legacy-index-concurrent-drop

Add test and fix issue with concurrent drop and usage of legacy index.