SQUASH CDentry: use MutationRef instead of MutationRef&

Signed-off-by: Greg Farnum <greg@inktank.com>
SQUASH Locker: use MutationRef instead of MutationRef&

Signed-off-by: Greg Farnum <greg@inktank.com>
SQUASH MDCache: use MutationRef instead of MutationRef&

Signed-off-by: Greg Farnum <greg@inktank.com>
SQUASH MDCache: fix a shared pointer cast

Signed-off-by: Greg Farnum <greg@inktank.com>
SQUASH MDCache: use a null_ref instead of NULL

Signed-off-by: Greg Farnum <greg@inktank.com>
SQUASH Locker: use MutationRef instead of MutationRef&

Signed-off-by: Greg Farnum <greg@inktank.com>
SQUASH Server: remove inappropriate put/get calls on MDRequestRef

Signed-off-by: Greg Farnum <greg@inktank.com>