changed explanation for group by clause
update changes to reflect changes from 0.42.2 release
forbid `RESET` stmt persistence keyword instead of just ignore with warning (removed from parser syntax)
added irc notifications to travis
Merge pull request #1140 from crate/m/group_by_synopsis

changed explanation for group by clause
various small doc improvements
clarify that group by columns must be present in result column list
add request handling for QueryShardRequest/Response
change ESSearchTask to use QueryThenFetch using QueryShardRequests
fix test flakiness
generate lucene Sort from symbols
support missing values in new InputFieldComparator