The channel will try to connect to the node for each message if a
connection has not yet been established or if the node has disconnected.

The only code change is the move of "if conn == nil" from connect()
to tryConnect(). It seems more logical to have this "guard check"
at the top of a method, rather than right after starting the
sendMsgs goroutine. There should be no semantic differences here,
but who knows... this code is a bit intricate.

Also adds some doc comment above some (internal) functions, mainly
for documentation purposes and some language polish and typos.

Added a guardclause to prevent goroutines being started more than once.

Also, the channel will only lock when trying to recreate a
nodestream, making it possible to retry when sending messages.

We can avoid passing the context to channel, since we anyway have
access to the relevant RawNode.

Since we now cancel pending messages, we have introduced a data
race that doesn't appear to have been triggered before.

To avoid having to support another test function, let's make it
unexported for now. Currently, we only use it in one place; maybe
we can harmonize it with some other helper.

It does not appear to be needed.