cbdeps: Add build for jemalloc

Add CMake build to download, compile and package jemalloc as a cbdep.

Change-Id: I49835bba58ed43270caf1dd93b01b85ae9098951
[Commit validation] ep-engine-gerrit.sh: Update to master branch, multple improvements

Cleanup and enhance the ep-engine commit validation build (in line
with similar changes to memcached, making it fit to run on every
ep-engine checkin. Makes a number of improvements:

- remove parallel-unfriendly functionality.
- Run tests in parallel
- Improve output formatting
- Echo main commands to console
- Remove cleanup of .repo - just slows down subsequent syncs
- Show test output on failure
- Generate xUnit results file
- Use ccache if available

Change-Id: I62d98695bad5ec1bb658e1b588bd4985c951082c