MB-30549: Use cb::RWLock for older clang builds
MB-30549: Use cb::RWLock for older clang builds

std::shared_timed_mutex is not available on all of our
builders, so switch to RWLock on those builds which
are lacking the required symbols.

Change-Id: I16f234ebae94eefa837b851d325b8c3ceb196fee
Reviewed-on: http://review.couchbase.org/97585
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine