privVal: Improve SocketClient network code (#1315)

Follow-up to feedback from #1286, this change simplifies the connection
handling in the SocketClient and makes the communication via TCP more
robust. It introduces the tcpTimeoutListener to encapsulate accept and
i/o timeout handling as well as connection keep-alive, this type could
likely be upgraded to handle more fine-grained tuning of the tcp stack
(linger, nodelay, etc.) according to the properties we desire. The same
methods should be applied to the RemoteSigner which will be overhauled
when the priv_val_server is fleshed out.

* require private key
* simplify connect logic
* break out conn upgrades to tcpTimeoutListener
* extend test coverage and simplify component setup

node/node.go

@@ -183,7 +183,7 @@ func NewNode(config *cfg.Config,
 			pvsc    = priv_val.NewSocketClient(
 				logger.With("module", "priv_val"),
 				config.PrivValidatorListenAddr,
-				&privKey,
+				privKey,
 			)
 		)