librbd/cache/pwl: add basic metrics to ImageCacheState

librbd/cache/pwl: add basic metrics to ImageCacheState

Add basic metrics to ImageCacheState and persist them, including
allocated_bytes, cached_bytes, dirty_bytes, free_bytes and hit/miss
info.

Leverage periodic_stats() timer to call update_image_cache_state.
In order to avoid outputting too much debug information, the original
statistics output log level is changed to 5.

Switch to json_spirit for encoding because encode_json encodes bool as
"true"/"false" string.

Remove rbd_persistent_cache_log_periodic_stats option because we need
to always update cache state.

[ idryomov: add cached_bytes and hits_partial; report misses and
  miss_bytes instead of respective totals; naming ]

Fixes: https://tracker.ceph.com/issues/50614
Signed-off-by: Yin Congmin <congmin.yin@intel.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>