s3-mdssvc: full Spotlight implementation

Implement all Spotlight RPC commands with the Tracker SPARQL async
query API.

Tracker uses glib for implemeting async tasks, we thus have to use a
glib mainloop for processing sheduled tasks in threads.

Signed-off-by: Ralph Boehme <rb@sernet.de>
s3-mdssvc: build utility spotlight2sparql

Signed-off-by: Ralph Boehme <rb@sernet.de>
s3-mdssvc: add mdssd RPC service daemon for mdssvc

Signed-off-by: Ralph Boehme <rb@sernet.de>
s3-mdssvc: add documentation for mdssvc and mdssd

Signed-off-by: Ralph Boehme <rb@sernet.de>