mod_multicast - Big rewrite: replaced loop with pool

During the past days I concentrated in improving a critical part of mod_multicast: the code that checks for protocol support on remote servers.

Originally all the processing for a user stanza was done in a single, large, procedural run. This included a loop that slowly checks XEP33 support for each remote server, sends the stanza accordingly, and then proceeds to the next server.

Now, the loop only sends the stanzas to the servers which support is already known. For the unknown servers, it only sends the iq:query disco#info request, but does not wait for an answer. Instead, the group of destinations related to that server (which are now considered 'Waiters') are temporarily stored in a 'Pool'. Eventually, a server answers, or an error is received; mod_multicast reads from the Pool the Waiters group and finally sends the stanza.

Future work: I must fix new bugs, related to the new code. Later I'll reconsider if the current Pool needs further changes. And finally, I can go back to the XEP33 problem with the addresses limits.