Simplify Swarm interface

[longfin]

Swarm currently exposes mainly three methods:

• InitAsync(): Initializes the internal objects needed for communication and connects to the registered peers.
• RunAsync(): Executes event loops to process the message.
• DisposeAsync(): Dispose the internal objects and requests the registered peers to delete it.

This design has the following problems.

• RunAsync() can be called before InitContextAsync(). but it raises NoSwarmContextException.
• it may leave Swarm in a strange state because RunAsync() does not cancel automatically after DisposeAsync().

So I suggest the following interface.

• StartAsync()
  • Initializes the internal objects.
  • Connects to the registered peers.
  • Executes event loops to process the message.
• StopAsync()
  • Disposes the internal objects.
  • Requests the registered peers to delete it.
  • Stops event loops.

[longfin]

A related issue(#72) was merged.