MB-100: make xmem error logs more user-friendly

QE mentioned it is not clear how to read the XMem stat in the
"XDCR errors" on UI. This commit will explain these stats to
make it more use-friendly. E.g, if xmem worker crashed due to
remote memcached errors, we will see something like this on UI:

"xmem stats: in batch of 500 docs: flushed: 460, rejected
(eexists): 10; remote memcached errors: enoent: 20, not-my-vb: 5,
invalid: 0, timeout: 5, others: 0."

Change-Id: I3d42832a589a21c5c072819516eeaec68b3f4d30
Reviewed-on: http://review.couchbase.org/28579
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>