FileStore: add config option to disable the wbthrottle

Backport: dumpling
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
WBThrottle: use fdatasync instead of fsync

Backport: dumpling
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>