Organizing plugins: scheduling and events

Plugins which close the RPC channel may conflict with other plugins.

```js
module.exports = function (server) {
  server.on('authorized', function (rpc) {
    replicate(server, rpc, function (err, progress) {
      if(err) return console.error(err)
      server.emit('replicated', progress)
      rpc.close(function () {}) // <-- this is an issue
    })
  })
}
```

Plugins should not directly control the lifecycle of a connection. This patch adds the scheduling API which allows plugins to register a job and specify a time limit. When all jobs are finished or timed-out, the server closes the channel.

```js
module.exports = function (server) {
  server.on('rpc:authorized', function(rpc) {
    // schedule 30 seconds to do replication
    server.schedule(rpc, 'replicate', 30, function(err, done) {
      if (err) return console.log('REPLICATE Failed to schedule session time', err)
      server.emit('replicate:start', rpc)
      replicate(server, rpc, function (err, progress) {
        if(err) {
          server.emit('replicate:fail', rpc, err)
          console.error(err)
        } else
          server.emit('replicate:finish', rpc, progress)
        done()
      })
    })
  })
}
```

This patch additionally structures the events using a namespace scheme of `'plugin:event'`. These are the current events:

 - `rpc:connect` - a new rpc session has been created
 - `rpc:peer` - an incoming rpc session has been created
 - `rpc:client` - an outgoing rpc session has been created
 - `rpc:close` - an rpc session has been closed
 - `rpc:authorized` - the rpc session has successfully authorized with the remote peer
 - `rpc:unauthorized` - the rpc session has failed to authorize with the remote peer
 - `replicate:start` - replicate plugin has started its job
 - `replicate:finish` - replicate plugin successfully finished its job
 - `replicate:fail` - replicate plugin failed its job
 - `gossip:connect` - gossip plugin created a new rpc session