Send ACK asap during handshake

[maskit]

Doing handshake stuff delays sending ACK, and the delay during handshake can be problematic because measured RTT on the peer gets too long.

[maskit]

Background: quicwg/base-drafts#2614

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. Marking it stale to flag it for further consideration by the community.