osd/ReplicatedPG: do watch effects only when change commits

osd/ReplicatedPG: do watch effects only when change commits

Do not tell the client their watch succeeded until it is durable and
visible to clients doing a notify.  This is less important in the initial
watch registration stage, but critical in the disconnect (and disconnect
+ reconnect) pipelining cases.

Simplify the watch_disconnects struct to have exactly the information
we need, and no more.  Note that the conn is not needed for disconnects
(and we don't have a req to provide one in the watch timeout op case).

Signed-off-by: Sage Weil <sage@redhat.com>