Rather than just closing the underlying network listener to stop our
HTTP servers, use the graceful shutdown procedure, waiting for any
in-process requests to finish.
This change makes it possible to run geth with JSON-RPC over HTTP and
WebSocket on the same TCP port. The default port for WebSocket
is still 8546.
geth --rpc --rpcport 8545 --ws --wsport 8545
This also removes a lot of deprecated API surface from package rpc.
The rpc package is now purely about serving JSON-RPC and no longer
provides a way to start an HTTP server.