s3-mdssvc: implement async Spotlight RPC

* dalloc: typesafe object container
* Spotlight blob marshalling and unmarshalling
* metadata attribute mapping
* lexer and parser for Spotlight queries
* Spotlight-RPC commands

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

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