Respond with first byte to extend timeout for early sampling requests

[dapplion]

As proposed by Codex, we can reduce lag when sampling in the happy path by sending DataColumnsByRoot requests immediately as we see the block and having the sampling provider hold the request until it gets the sample.

This mechanism is already implemented in our end, both the eager sending, and the holding of requests. However, we are bound by the ReqResp timeout, which may be too short depending on how wide the sampling window is. We may artificially extend the deadline by sending the first byte of data.

[dapplion]

The TTFB mechanism (time to first byte) has been deprecated. So we no longer need to send the first byte, just hold the request and expect the requester timeout to be enough.

• p2p: Deprecate TTFB, RESP_TIMEOUT, introduce rate limiting recommenda… ethereum/consensus-specs#3767