Fix maxMessageSize not effective even if aligned with broker (#381)

### Motivation
- issue 1
    If broker updated `maxMessageSize`, client's `maxMessageSize` will be the same after handshaking.
    However, client still use the default `maxMessageSize` while reading command from connection.
    Lead to consumer can't receive message which payload exceed 5MB.

- issue 2
   According to [PIP-36](https://github.com/apache/pulsar/wiki/PIP-36%3A-Max-Message-Size), default *Size should be:
   ```
   maxMessageSize = 5MB
   framePaddingSize = 10KB
   maxFrameSize = maxMessageSize + framePaddingSize
   ```
   But they two are confused currently：
   ```
   maxFrameSize = 5MB
   framePaddingSize = 10KB
   maxMessageSize = maxFrameSize - framePaddingSize
   ```

### Modifications

- Use the aligned `maxMessageSize` instead of the default value.
- Correct `maxMessageSize` default value.



### Verifying this change

- [x] Make sure that the change passes the CI checks.

pulsar/internal/commands.go

@@ -28,13 +28,13 @@ import (
 )
 
 const (
-	// MaxFrameSize limit the maximum size that pulsar allows for messages to be sent.
-	MaxFrameSize = 5 * 1024 * 1024
+	// MaxMessageSize limit message size for transfer
+	MaxMessageSize = 5 * 1024 * 1024
 	// MessageFramePadding is for metadata and other frame headers
 	MessageFramePadding = 10 * 1024
-	// MaxMessageSize limit message size for transfer
-	MaxMessageSize        = MaxFrameSize - MessageFramePadding
-	magicCrc32c    uint16 = 0x0e01
+	// MaxFrameSize limit the maximum size that pulsar allows for messages to be sent.
+	MaxFrameSize        = MaxMessageSize + MessageFramePadding
+	magicCrc32c  uint16 = 0x0e01
 )
 
 // ErrCorruptedMessage is the error returned by ReadMessageData when it has detected corrupted data.

pulsar/internal/connection_reader.go

@@ -73,7 +73,7 @@ func (r *connectionReader) readSingleCommand() (cmd *pb.BaseCommand, headersAndP
 
 	// We have enough to read frame size
 	frameSize := r.buffer.ReadUint32()
-	if frameSize > MaxFrameSize {
+	if r.cnx.maxMessageSize != 0 && int32(frameSize) > (r.cnx.maxMessageSize+MessageFramePadding) {
 		r.cnx.log.Warnf("Received too big frame size. size=%d", frameSize)
 		r.cnx.TriggerClose()
 		return nil, nil, errors.New("Frame size too big")