[Commit validation] memcached-gerrit.sh: Update to master branch, multple improvements

Cleanup and enhance the memcached commit validation build, making it
fit to run on every memcached 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

This script (checkout proposed patchm, build whole tree & run
memcached unit tests) now takes approx 2m40s.

Change-Id: Id3923f94c6dbc867150041ed6dfcdf3a86803e16
Reviewed-on: http://review.couchbase.org/42766
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Dave Rigby <daver@couchbase.com>