secureSession.Read([]byte) seems to behave like io.ReadFull

[raulk]

On studying the Read([]byte) method, it seems like it strives to fill the buffer, which is contrary to the contract of io.Reader.

We should:

• vacate the queued buffer ASAP whether the provided buffer is larger or smaller than the queued data, it doesn't matter, returning the effective number of bytes returned.
• never eagerly block on further incoming data, if we already have data to return.