changed explanation for group by clause
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
fix bulk args validation

detected type missmatch if one value was null
sync changelog with 0.42 branch
prepare release 0.43.0
added graceful_stop cluster settings
documentation for graceful_stop cluster settings
implemented rounting.allocation.enable setting
test fixup
changed implementation of global settings
added cluster upgrade settings to crate.yml
added docu for extended cluster settings
moved settings documentation from reference to documentation
added missing configuration settings description
to crate.yml
implemented mostly all dynamic cluster setting expressions
fixed setting object settings
added missing settings to documentation