RPCServer: close chan when Serve returns

[fairclothjm]

Ensure the RPCServer Serve function closes the DoneCh when it returns. Add a test that will hang if we don't close the channel.

Fixes #216

Log diff after changes using the basic example

diff --git a/rpc.log b/rpc.log
index c7237c7..b8ca295 100644
--- a/rpc.log
+++ b/rpc.log
@@ -7,7 +7,6 @@
 [DEBUG] plugin: using plugin: version=1
 [DEBUG] plugin.greeter: message from GreeterHello.Greet: timestamp=NORMALIZED
 Hello!
-[DEBUG] plugin.greeter: NORMALIZED [ERR] plugin: stream copy 'stderr' error: stream closed
 [DEBUG] plugin.greeter: NORMALIZED [DEBUG] plugin: plugin server: accept unix NORMALIZED: use of closed network connection
 [INFO]  plugin: plugin process exited: path=./plugin/greeter pid=NORMALIZED
 [DEBUG] plugin: plugin exited

Test output

Wed Oct 12 09:39:19 CDT 2022
--- PASS: TestClient_testInterfaceReattach (2.01s)
--- PASS: TestClient (0.01s)
--- PASS: TestClient_killStart (0.01s)
--- PASS: TestClient_testCleanup (0.01s)
--- PASS: TestClient_testInterface (0.01s)
--- PASS: TestClient_grpc_servercrash (0.01s)
--- PASS: TestClient_grpc (0.01s)
--- PASS: TestClient_grpcNotAllowed (0.01s)
--- PASS: TestClient_grpcSyncStdio (0.03s)
--- PASS: TestClient_cmdAndReattach (0.00s)
--- PASS: TestClient_reattach (1.01s)
--- PASS: TestClient_reattachNoProtocol (1.01s)
--- PASS: TestClient_reattachGRPC (1.01s)
--- PASS: TestClient_reattachNotFound (0.01s)
--- PASS: TestClientStart_badVersion (0.01s)
--- PASS: TestClientStart_badNegotiatedVersion (0.01s)
--- PASS: TestClient_Start_Timeout (0.05s)
--- PASS: TestClient_Stderr (0.02s)
--- PASS: TestClient_StderrJSON (0.02s)
--- PASS: TestClient_textLogLevel (0.02s)
--- PASS: TestClient_Stdin (0.09s)
--- PASS: TestClient_SecureConfig (0.10s)
--- PASS: TestClient_TLS (0.03s)
--- PASS: TestClient_TLS_grpc (0.02s)
--- PASS: TestClient_secureConfigAndReattach (0.00s)
--- PASS: TestClient_ping (0.01s)
--- PASS: TestClient_wrongVersion (0.01s)
--- PASS: TestClient_legacyClient (0.01s)
--- PASS: TestClient_legacyServer (2.01s)
--- PASS: TestClient_versionedClient (0.02s)
--- PASS: TestClient_mtlsClient (0.06s)
--- PASS: TestClient_mtlsNetRPCClient (0.05s)
--- PASS: TestClient_logger (0.43s)
--- PASS: TestClient_logStderr (0.00s)
--- PASS: TestBasicError_ImplementsError (0.00s)
--- PASS: TestBasicError_MatchesMessage (0.00s)
--- PASS: TestNewBasicError_nil (0.00s)
--- PASS: TestGRPCClient_App (0.00s)
--- PASS: TestGRPCConn_BidirectionalPing (0.00s)
--- PASS: TestGRPCC_Stream (0.00s)
--- PASS: TestGRPCClient_Ping (0.00s)
--- PASS: TestGRPCClient_Reflection (0.00s)
--- PASS: TestHelperProcess (0.00s)
--- PASS: TestClient_App (0.00s)
--- PASS: TestClient_syncStreams (0.10s)
--- PASS: TestServer_testMode (0.00s)
--- PASS: TestServer_RPC (0.00s)
--- PASS: TestServer_testMode_AutoMTLS (0.08s)
--- PASS: TestRmListener_impl (0.00s)
--- PASS: TestRmListener (0.00s)
--- PASS: TestProtocolSelection_no_server (0.00s)
--- PASS: TestServer_testStdLogger (0.00s)
ok  	github.com/hashicorp/go-plugin	9.010s

[swenson]

LGTM; makes sense.

I'm not 100% sure it matters in most normal use cases sense in non-test scenarios, the Serve() function will call os.Exit().