add swarm manager

[dch]

I pulled last 3 items from #22 into own ticket.

I'm assuming this will be another gen_server + supervisor.

api

• [x] given a swarm id (hash), return the managing swarm process
• [x] given a channel, return the managing swarm process
• [x] separate to channel_manager, get swarm options from that process
• interim we'll have a stubbed convenience function in channel_manager to do that

  spec

-spec swarm:find_swarm_worker(ppspp_options:root_hash()) ->
   swarm_worker:worker_id().
-spec swarm:find_swarm_worker(ppspp_channel:channel()) ->
   swarm_worker:worker_id().
-spec swarm:get_swarm_options(ppspp_channel:channel()) ->
    ppspp_options:options().
-spec swarm:get_swarm_options(ppspp_options:root_hash()) ->
    ppspp_options:options().

notes

• a channel is unique for (belongs to) a single peer, and a single endpoint (client peer), and a single swarm (root hash)
• channels are for the moment assumed to be unique across an erlang cluster (global registration)
• each swarm holds its own state about the required options
• the inquiring process can then query the swarm to retrieve specific options
• should track the owning process for a channel and clean up if it exits
• a future swarm_worker will hold its own swarm options, and register itself
• it will start the peer_worker and pass the swarm options through

[dch]

implemented most of these directly in swarm_worker for the moment, except channel->swarm_id which is in channel, because it makes more sense wrt types.