cephfs-tool: add async I/O support with configurable queue depth

cephfs-tool: add async I/O support with configurable queue depth

Add --async-io to use ceph_ll_nonblocking_readv_writev for read and write
workers instead of synchronous ceph_read/ceph_write. Add --queue-depth to
limit outstanding async I/Os per thread via a counting semaphore and
pre-allocated AsyncIOContext pool. Async write uses ceph_ll_create; async
read uses ceph_ll_walk + ceph_ll_open and defers inode release until all
async ops including readahead complete. Reject --async-io with
--per-thread-mount since async workers require a shared mount.

Fixes: https://tracker.ceph.com/issues/76183
Signed-off-by: Edwin Rodriguez <edwin.rodriguez1@ibm.com>