Allow the user to set a custom timeout for dialing a host

[jkawamoto]

In addition to #90, it would be helpful if the user can set the timeout for dialing a host to a custom value.

[lukechampine]

#90 was pretty simple, but I'm less sure how this should be handled.

Technically there are two timeouts here: one for dialing the host, and another for performing the initial handshake. Sometimes you want to use a single timeout that covers both operations, and other times you want to use distinct timeouts. So the API could get pretty ugly.

Perhaps the best approach would be to add a NewSessionFromConn function that takes a conn already initialized by the caller. You would use it like so:

conn, err := net.DialTimeout("tcp", string(hostIP), customDialTimeout)
if err != nil {
	return nil, err
}
conn.SetDeadline(time.Now().Add(customHandshakeTimeout))
s, err := proto.NewSessionFromConn(conn, hostKey, currentHeight)
if err != nil {
	return nil, err
}

This would also allow the use of e.g. DialContext if you want to use contexts to manage cancellation. And since Conn is an interface, you could wrap it if desired, much like how statsConn works. (Hmm, maybe statsConn could be moved out of proto entirely...)

[jkawamoto]

Context-based cancellation sounds good. It'd be handly If PseudoKV #67 supports it. On the other hand, I think it's not good for PseudoFS because adding ctx to each method makes PseudoFS less compatible with the typical FileSystem interface.