abci: modify Client interface and socket client (#5673)

`abci.Client`:

- Sync and Async methods now accept a context for cancellation
    * grpc client uses context to cancel both Sync and Async requests
    * local client ignores context parameter
    * socket client uses context to cancel Sync requests and to drop Async requests before sending them if context was cancelled prior to that

- Async methods return an error
    * socket client returns an error immediately if queue is full for Async requests
    * local client always returns nil error
    * grpc client returns an error if context was cancelled before we got response or the receiving queue had a space for response (do not confuse with the sending queue from the socket client)

- specify clients semantics in [doc.go](https://raw.githubusercontent.com/tendermint/tendermint/27112fffa62276bc016d56741f686f0f77931748/abci/client/doc.go)

`mempool.TxInfo`

- add optional `Context` to `TxInfo`, which can be used to cancel `CheckTx` request

Closes #5190

abci/example/example_test.go

@@ -1,6 +1,7 @@
 package example
 
 import (
+	"context"
 	"fmt"
 	"math/rand"
 	"net"
@@ -13,8 +14,6 @@ import (
 
 	"google.golang.org/grpc"
 
-	"golang.org/x/net/context"
-
 	"github.com/tendermint/tendermint/libs/log"
 	tmnet "github.com/tendermint/tendermint/libs/net"
 
@@ -45,7 +44,7 @@ func TestGRPC(t *testing.T) {
 }
 
 func testStream(t *testing.T, app types.Application) {
-	numDeliverTxs := 20000
+	const numDeliverTxs = 20000
 	socketFile := fmt.Sprintf("test-%08x.sock", rand.Int31n(1<<30))
 	defer os.Remove(socketFile)
 	socket := fmt.Sprintf("unix://%v", socketFile)
@@ -53,9 +52,8 @@ func testStream(t *testing.T, app types.Application) {
 	// Start the listener
 	server := abciserver.NewSocketServer(socket, app)
 	server.SetLogger(log.TestingLogger().With("module", "abci-server"))
-	if err := server.Start(); err != nil {
-		require.NoError(t, err, "Error starting socket server")
-	}
+	err := server.Start()
+	require.NoError(t, err)
 	t.Cleanup(func() {
 		if err := server.Stop(); err != nil {
 			t.Error(err)
@@ -65,9 +63,8 @@ func testStream(t *testing.T, app types.Application) {
 	// Connect to the socket
 	client := abcicli.NewSocketClient(socket, false)
 	client.SetLogger(log.TestingLogger().With("module", "abci-client"))
-	if err := client.Start(); err != nil {
-		t.Fatalf("Error starting socket client: %v", err.Error())
-	}
+	err = client.Start()
+	require.NoError(t, err)
 	t.Cleanup(func() {
 		if err := client.Stop(); err != nil {
 			t.Error(err)
@@ -101,22 +98,24 @@ func testStream(t *testing.T, app types.Application) {
 		}
 	})
 
+	ctx := context.Background()
+
 	// Write requests
 	for counter := 0; counter < numDeliverTxs; counter++ {
 		// Send request
-		reqRes := client.DeliverTxAsync(types.RequestDeliverTx{Tx: []byte("test")})
-		_ = reqRes
-		// check err ?
+		_, err = client.DeliverTxAsync(ctx, types.RequestDeliverTx{Tx: []byte("test")})
+		require.NoError(t, err)
 
 		// Sometimes send flush messages
-		if counter%123 == 0 {
-			client.FlushAsync()
-			// check err ?
+		if counter%128 == 0 {
+			err = client.FlushSync(context.Background())
+			require.NoError(t, err)
 		}
 	}
 
 	// Send final flush message
-	client.FlushAsync()
+	_, err = client.FlushAsync(ctx)
+	require.NoError(t, err)
 
 	<-done
 }