Publish in batch

[twoeths]

Motivation
There is a missed attestation issue described in ChainSafe/lodestar#4600. When lodestar wants to publish a gossip message, it wants to spread to the network asap

Description
Right now the steps to publish a message are as below:

• Select peers to publish, this is usually about 20 peers (as in lodestar)
• For each peer:
  • Form a new rpc for that peer with cached control messages of that peer
  • Encode message to bytes using protobuf
  • Encode to length prefix
  • Send to raw stream

The proposal is to send in batch without checking cached control messages

• Select peers to publish
• Encode messages to bytes using protobuf, then encode length prefix
• Send that bytes data to all raw stream

Something to consider

• Is it against the spec @wemeetagain
• Should we add a new api or add a new flag to publish() method? Main thing is not to introduce a breaking change
• Does it have any side effect to control messages? Since we have to forward messages very frequently, and we also have heartbeat, I suppose "no" optimistically

[wemeetagain]

This proposal is ok according to the spec.
Lets add a new flag to PublishOpts