RelationKey: fix postgres length check to be 2 longer

We were counting the opening and close quotes, and we shouldn't have.
ProfilingLogger: create useful index for sending queries

We do not order by or group by nanoTime. we group by query/fragment and
query/fragment/destWorker. Thus, create the index on
query/fragment/destWorker which will be useful for both.
Server: remove null characters in profiling plan strings

Old value: "download profiling histogram (query=\u000059487\u0000,
fragment=\u00003\u0000, range=[\u00000, 2797149853,
4757057\u0000]\u0000)"

is not what we want...
Server: add duration to network link stats
ProfilingLogger: tweak sent index further

See discussion on https://github.com/uwescience/myria/pull/668