JCBC-463: Harden shutdown procedure in CouchbaseClient

Motivation
----------
Lingering bucket streaming threads are reported, this could be the case
if something is preventing them from being shut down.

Modifications
-------------
Split up the spymemcached IO and view IO shutdown in two different
try/catch blocks, to make sure the other part always runs, even if
something goes wrong in the other.

This change goes hand-in-hand with related shutdown hardening changes
in spymemcached directly.

Result
------
Shutting down of the config threads now also happens if an exception
is raised during memcached IO shutdown.

Change-Id: I65bbe9ab1b5c27333012268cd3aafa5d8aa528b4
Reviewed-on: http://review.couchbase.org/37726
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>