Ignore unused field warning caught by golangci-lint v1.51.2

Reworked internal/conn handshake abstractions. (ServiceWeaver#54)

Recall that when an envelope and a weavelet establish a connection to
one another, they perform a simple handshake. The envelope sends a
protos.Weavelet, and the weavelet acks it.

Before this PR, the abstractions to perform this handshake were a bit
shaky. You would construct an envelope conn and then call SendWeaveletInfoRPC:

    e := conn.NewEnvelopeConn(...)
    e.SendWeaveletInfoRPC()

However, this wouldn't work because the RPC depends on e.Run() having
been called. So you need to spawn a goroutine to run e.Run(). On the
weavelet side, you have to call GetWeaveletInfo before you call w.Run():

    w := conn.NewWeaveletConn(...)
    w.GetWeaveletInfo()

If you forgot any of these calls or did them in the wrong order, things
would hang or fail.

This PR reworks the abstractions so that you can't forget to perform the
handshake. Now, conn.NewWeaveletConn automatically sends the
protos.Weavelet and does not wait for an ack. Simmilarly,
conn.NewWeaveletConn blocks until it receives the protos.Weavelet. This
makes it much harder to forget or mess up the handshake.

This PR is part of a larger effort to clean up the runtime protos and
pipe API. There is still a lot of cruft. For example, the EnvelopeConn
methods will hang unless you call Run(). A lot of the protos also pass
redundant information. I'll try to clean a lot of this up in future PRs.