NCBC-721: Ensure Increment Wraps when it Overflows

This patch fixes a bug where the Increment interface incorrectly was
declared with a signed long instead of an unsigned long, which resulted in
an overflow exception before the value wrapped back around to zero.

Change-Id: Iee17c7dc0df197036e701c9bdb6de98da5b1918a
Reviewed-on: http://review.couchbase.org/43281
Reviewed-by: Simon Baslé <simon@couchbase.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>