Avoid allocating new slice for data on every Receive()

[jonhoo]

In MCRequest.Receive, a new buffer is allocated for every receive for the body of the request:

buf := make([]byte, klen+elen+bodyLen)

It would be great if this function could instead accept a buffer as an argument so that buffers could be re-used across requests. The code could use cap(buf) to ensure that the buffer is long enough, and buf = buf[:klen+elen+bodyLen] to limit its length. Only if it buf is too small should it allocate its own (and potentially warn the caller that the buffer was not used).