Make MSleep (sleep) never constant so it get executed during query
processing not during parse, so interrupt is handled correctly.
Make MSleep#doEvaluate() check the session canceled on interrupt and
throw a QueryCanceledException if so.
merge tjoneslo: Msleep not constant

Merge pull request #1046 from tjoneslo/sql-layer/msleep-not-constant
https://github.com/FoundationDB/sql-layer/pull/1046