osd: magically fall back to leveldb for xattrs
see the grief for example in #4902
add some librados api tests for large xattrs, many xattrs.
os/FileStore: automatically enable 'filestore xattr use omap' as needed
Automatically enable the 'filestore xattr use omap' option if the fs
does not appear to handle large xattrs on its own.
This makes for a more pleasant use experience as they are not told to
enable something that we already know they must enable in order to