encryptedTCPSender.Send() holds a lock over a TCP send

[bboreham]

Lock is not released until the Send() returns.

Taken together with #125 this means it can hold a lock forever.

Sender and receiver each maintain state including a sequence number, so if we unlock before calling Send() it's possible that messages get out of order.

Seen at https://github.com/weaveworks/weave/issues/3762

[bboreham]

I am now thinking this is not a bug, on the basis:

• there is one lock per outgoing socket
• the Send() on the socket will block all callers when the buffer is full

therefore it doesn't make any difference if we block earlier on the lock.