os/bluestore: Refactor of WAL-v2 BlueFS

os/bluestore: Refactor of WAL-v2 BlueFS

Main changes:
- remove bluefs_wal_header_t; we no longer use denc directly,
  instead we read length and marker directly
- get rid of wal_limit; this is replaced with splitting
  WAL_V2=>{WAL_V2,WAL_V2_FIN}. WAL_V2 means there could be more wal data,
  WAL_V2_FIN means file was orderly closed = no more data.
- modified wal_marker_t (previously WALMarker) to be calculated in
  endiness-agnostic way.
- replaced _wal_update_size with _wal_index_file; it does a similar
  function, but is less convoluted now
- simplified _wal_read
- now BlueFS::close_writer() flushes data
- BlueRocksEnv's BlueRocksWritableFile::truncate() now does the action

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>