ceph-for-5.7-rc1

The main items are:

- support for asynchronous create and unlink (Jeff Layton).  Creates
  and unlinks are satisfied locally, without waiting for a reply from
  the MDS, provided the client has been granted appropriate caps (new
  in v15.y.z ("Octopus") release).  This can be a big help for metadata
  heavy workloads such as tar and rsync.  Opt-in with the new nowsync
  mount option.

- multiple blk-mq queues for rbd (Hannes Reinecke and myself).  When
  the driver was converted to blk-mq, we settled on a single blk-mq
  queue because of a global lock in libceph and some other technical
  debt.  These have since been addressed, so allocate a queue per CPU
  to enhance parallelism.

- don't hold onto caps that aren't actually needed (Zheng Yan).  This
  has been our long-standing behavior, but it causes issues with some
  active/standby applications (synchronous I/O, stalls if the standby
  goes down, etc).

- .snap directory timestamps consistent with ceph-fuse (Luis Henriques)
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