rpc: keep track of acceptors, and cancel them in StopRPCThreads
rpc: keep track of acceptors, and cancel them in StopRPCThreads

Fixes #4156.

The problem is that the boost::asio::io_service destructor
waits for the acceptors to finish (on windows, and boost 1.55).

Fix this by keeping track of the acceptors and cancelling them before
stopping the event loops.

wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin