ENG-5043 fail elastic clear index if it is not empty
ENG-5043 merged from elastic. Resolved conflict in src/ee/storage/ElasticContext.cpp
ENG-5043 addressed Ariel's code review comments
ENG-5043 addressed Ariel's code review comments (take II)
Merge pull request #770 from VoltDB/ENG-5043

ENG-5043 fail elastic clear index if it is not empty