mds: identify slave requests with reqid + attempt number

mds: identify slave requests with reqid + attempt number

We need to distinguish between different attempts to process a request, or
else we can get annoying races in the slave request handling code.  E.g.,

- request sent to mds A
- A authpins items on B, B registered slave_request
- A forwards request to C, sends slave finish to B
- C receives request, sends authpin slave request to B
- B receives C's authpin request, discards (*)
- B receives A's finish, closes slave request

First we just add tracking of the attempt number.

Signed-off-by: Sage Weil <sage@newdream.net>