FileStore: add config option to disable the wbthrottle

Signed-off-by: Samuel Just <sam.just@inktank.com>
WBThrottle: use fdatasync instead of fsync

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: don't maintain log_keys_debug if the config is disabled

Signed-off-by: Samuel Just <sam.just@inktank.com>