use extended labels for message headers instead of 3-char labels

the 3-char labels scheme was chosen during the earlier stages of this project
when only get/set/del commands were planned. As of today, considering the number
of available commands and the chance to add new ones, the short names started becoming
confusing and a bit cryptic

- also allow both shardcache_client_get_async() and read_message_async() to be interrupted
  by their callbacks (if -1 is returned by the callback, the asynchronous get will be interrupted)
shardcache_get_async() is now available in the perl-xs client library