prevent unbounded number of goroutines for connection shutdown
prevent unbounded number of goroutines for connection shutdown

We need Connection.Shutdown to be non-blocked, and the way we achieved
this is by spawning a goroutine. However, there can be a potentially
unbounded number of Connection.Shutdown invocations. For example, if
the TCP connection has gone bad then any Connection.SendProtocolMsg
invocation will end up invoking Connection.Shutdown. There can be many
such invocations until the Connection has been removed from the
various data structures (e.g. LocalPeer.connections and
GossipSender.senders). Hence we can end up with with an unbounded
number of spawned goroutines.

To fix this, we introduce a one-element error channel on the
connection. Connection.Shutdown does a non-blocking send on that, and
the Connection.actorLoop reads from it and terminates.

This will throw away all but the first error encountered, but that is
fine and not much different from what was happening before (where we'd
throw away all but one of the errors encountered).

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