objecter: separate out linger_read() and linger_mutate()

objecter: separate out linger_read() and linger_mutate()

A watch is a mutation, while a notify is a read.  The mutations need to
pass in a proper snap context to be fully correct.

Also, make the WRITE flag implicit so the caller doesn't need to pass it
in.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 6c08c7c1c6d354d090eb16df279d4b63ca7a355a)