Time = 0 is valid time in ngtcp2

so that application is not bothered by another cryptic function.

Still we need to add additional handshake timeout.

Coalesces Initial and Handshake from server on probe.  Include
occasional PING in non-ACK eliciting packets.

Major rework of loss detection mechanism.  It turned out getting it
done is very hard.

It now defaults to 2 and includes CID exchanged during handshake.

The changed semantics will be implemented in the later commits.