save and update metadata configuration

[jrse]

Task of this issue is to create two new configuration settings in the dovecot configuration.
Both new settings should have a comma separated list of XAttributes.
The name of a XAttribute should be readable and unique. Currently all possible XAttributes are
defined in rados-types.h. A mapping function should be created to map the config XAttribute name to the
rbox_metadata_key.

We need two new settings to distinguish between mutable and immutable attributes.
Mutable attributes should be updated when changed.

To make this work we need to modify the read / save metadata functions in rbox_save / rbox_read,
the rbox_copy module, the rbox_rebuild methods to restore all available metadata and
additionally the rmb tool needs to be modified to display/set the attributes.