Use read() sync-plugin call to get entire-entry when merging to avoid redundancy

[dgollub]

Some sync-plugins can/could retrieve single records/entries from the peer - e.g. via a read() sync-plugin function. This could be also used (optional) to avoid storing the entire entry for merging missing-capabilities, instead of becoming redundant when storing it always in Archive.

The Archive should only be used if the plugin doesn't provide any read() call.

This requires also that the format-plugin, which is requested by the resource configuration of the sync-plugin, is able to merge/demerge.