Add ability to simulate network partitions.

New methods:

  * Bud#connect_channels
  * Bud#disconnect_channels
  * BudChannel#connect
  * BudChannel#disconnect

When a channel is disconnected, all tuples derived into the channel locally (via
<~ from within the disconnected Bud instance) are silently dropped on the floor;
similarly, inbound tuples from other instances that are destinated for a
disconnected channel are also silently dropped.