Require tcmalloc in memcached

All of our builds have used tcmalloc for a while and there is really
no good reason for us to use the standard allocator. This change
makes tcmalloc a requirement and removes all of the dlsym code.

Change-Id: I8b50f46bec115279c7ecf7b3e61b05053bf9239b
Reviewed-on: http://review.couchbase.org/30546
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>