store: Split out common JSON code into a separate helper function.
plugin: Make sdb_plugin_info_t public.
plugin: Added sdb_plugin_current().

This function returns the plugin information for the plugin from which the
current call into the core originated.
store: Store the backend of each stored object.

The backends are stored as a list of the plugin names of the plugins providing
the respective object.
store: Include backend information in the JSON serialization.
Merged branch 'master' of git://git.tokkee.org/sysdb.