cephfs-tool: Add support for `--async-write-fsync` and `--shared-file` options

Add --async-write-fsync and --shared-file benchmark options.

--async-write-fsync sets ceph_ll_io_info.fsync on each async write to model
Ganesha-style write+commit and skips the post-phase ceph_ll_fsync when enabled.
--shared-file directs all worker threads at one file to stress client_lock
contention on a single inode.  Both options are recorded in JSON/text output.

Signed-off-by: Edwin Rodriguez <edwin.rodriguez1@ibm.com>