os/DBObjectMap, FileStore: omap_clear should not remove xattrs

Prevously, FileStore::_omap_clear() used ObjectMap::clear(), which
incorrectly also blasts any stored xattrs.  Instead, add
ObjectMap::clear_keys_header() to handle this case efficiently.

Fixes: #7065
Fixes: #7135
Signed-off-by: Samuel Just <sam.just@inktank.com>
src/test/librados/misc.cc: verify that omap_clear doesn't blast xattr

Signed-off-by: Samuel Just <sam.just@inktank.com>
src/test/ObjectMap: test clear_keys_header() call

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