os, osd, tools: Add backportable compatibility checking for sharded objects

OSD
  New CEPH_OSD_FEATURE_INCOMPAT_SHARDS
FileStore
  NEW CEPH_FS_FEATURE_INCOMPAT_SHARDS
  Add FSSuperblock with feature CompatSet in it
  Store sharded_objects state using CompatSet
  Add set_allow_sharded_objects() and get_allow_sharded_objects() to FileStore/ObjectStore
  Add read_superblock()/write_superblock() internal filestore functions
ceph_filestore_dump
  Add OSDsuperblock to export format
  Use CompatSet from OSD code itself in filestore-dump tool
  Always check compatibility of OSD features with on-disk features
  On import verify compatibility of on-disk features with export data
  Bump super_ver due to export format change

Backport: dumpling, cuttlefish

Signed-off-by: David Zafman <david.zafman@inktank.com>
(cherry picked from commit c6b83180f9f769de27ca7890f5f8ec507ee743ca)

Conflicts:

	src/os/FileStore.cc
	src/os/FileStore.h
	src/osd/OSD.cc

Excluded from cherry-pick:
  Didn't add set_allow_sharded_objects() and get_allow_sharded_objects() to FileStore/ObjectStore
  Didn't add code to check for incomplete transition to sharded objects in ceph-filestore-dump