Change tid_t to ceph_tid_t to avoid conflicts with a popular type

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #1500 from ceph/wip-tid_t

Change tid_t to ceph_tid_t to avoid conflicts with a popular type

Reviewed-by: Sage Weil <sage@inktank.com>