implemented both 'add' and 'exists' commands

'add' is meant as in memecached jargon : 'set-if-not-exists'

Using these commands won't cause the data to be loaded into the cache

(NOTE: only the 'get*' family and the 'head' commands load the data into the cache)
'add' and 'exists' exposed by the client library as well

and completed the implementation in shardcache.c which was still calling
send_to_peer() also when if-not-exists was set
(should have called add_to_peer() instead)
bugfix

forgot to set the return-code in last commit
documented the newly added functions in both the server and the client APIs