rpc/jsonrpc/server: ws server optimizations (#5312)

* docs: goleveldb is much more stable now

Refs https://github.com/syndtr/goleveldb/issues/226#issuecomment-682495490

* rpc/core/events: make sure WS client receives every event

previously, if the write buffer was full, the response would've been
lost without any trace (log msg, etc.)

* rpc/jsonrpc/server: set defaultWSWriteChanCapacity to 1

Refs #3905
Closes #3829

setting write buffer capacity to 1 makes transactions count per block
more stable and also reduces the pauses length by 20s.

before: https://github.com/tendermint/tendermint/issues/3905#issuecomment-681854328 net.Read - 20s
after: net.Read - 0.66s

* rpc/jsonrpc/server: buffer writes and avoid io.ReadAll during read

rpc/client/main_test.go

@@ -18,6 +18,7 @@ func TestMain(m *testing.M) {
 	if err != nil {
 		panic(err)
 	}
+
 	app := kvstore.NewPersistentKVStoreApplication(dir)
 	node = rpctest.StartTendermint(app)
 
@@ -25,5 +26,6 @@ func TestMain(m *testing.M) {
 
 	// and shut down proper at the end
 	rpctest.StopTendermint(node)
+	_ = os.RemoveAll(dir)
 	os.Exit(code)
 }