SPY-156: Expose more async mutate methods.

Motivation
----------
More flexible incr/decr methods are exposed as sync variants than
with their async counterparts. This patch is an effort to bring them
on par (almost) completely.

Modifications
-------------
More async variants have been exposed, which were already supported
through the private asyncMutate method. Note that since the ascii
protocol doesn't support defaults and expiration for incr/decr, those
return an UnsupportedOperationException, because it is nontrivial to
fix. Please refer to the sync variants if you need that.

Result
------
When the binary protocol is used, much more asyncIncr/asyncDecr methods
are exposed, explicitly helping with default values and expiration.

Change-Id: I4403b14d6146afb325afa9c239da28b55e89e935
Reviewed-on: http://review.couchbase.org/34257
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>