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