ssl_c: Plug ownership/callback leaks

See inline comments and modifications for the painful details. In short:

- Always ensure pending write buffers are explicitly freed (i.e.
  returned to the lcbio_CTX/lcbio_SOCKET level) when the CSSL object is
  closed.
- Always trigger the user callback if a user-request is effective for a
  read and we have an error.

Change-Id: Id9ffd96d66505b3ec6f9d6ec479fe504432caa52
Reviewed-on: http://review.couchbase.org/43201
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>