ChainSyncClient: respond to ControlMessageSTM

[mrBliss]

Fixes #2528.

[mrBliss]

@coot The ChainSync client is the only client developed by consensus. All other clients are in the network layer. We have some servers (in this folder https://github.com/input-output-hk/ouroboros-network/tree/master/ouroboros-consensus/src/Ouroboros/Consensus/MiniProtocol), but these protocols (ChainSync and BlockFetch) don't allow the server to gracefully terminate the connection.

[mrBliss]

@coot CI fails because of a test failure (https://hydra.iohk.io/build/3901172/nixlog/1):

    bidirectional:                                             FAIL (1.09s)
      *** Failed! Exception: 'ExceptionInHandler 127.0.0.1:63956 MuxError (MuxIOException Network.Socket.recvBuf: resource vanished (Connection reset by peer)) "(recv errored)"' (after 54 tests):
      ClientAndServerData {responderAccumulatorFn = <fun>, hotResponderAccumulator = 2, warmResponderAccumulator = 52, establishedResponderAccumulator = 18, hotInitiatorRequests = [[-23,-69,14,84,-69,66,-96,89,-21,-76]], warmInitiatorRequests = [], establishedInitiatorRequests = [[81,-75,60],[50,-13,36,90,42,-13,44,89],[-90,37,13,-8,-69]]}
      ClientAndServerData {responderAccumulatorFn = <fun>, hotResponderAccumulator = 45, warmResponderAccumulator = 9, establishedResponderAccumulator = -23, hotInitiatorRequests = [[83,35,-53,-88,90,-7,21,26]], warmInitiatorRequests = [[-69],[]], establishedInitiatorRequests = [[10,-35,-39,48,60,44,98],[98,54,35,6,-56,43,-87],[-2,73,-30,66,-99,58]]}
      Use --quickcheck-replay=771777 to reproduce.

[coot]

LGTM