introduce a quic.Config.HandshakeIdleTimeout, remove HandshakeTimeout

[marten-seemann]

Fixes #1681.

There's a subtle difference between a handshake timeout and the handshake idle timeout: The handshake timeout fires a fixed time after the handshake started, and kills the connection if the handshake hasn't completed by that time.
The handshake idle timeout functions like the normal idle timeout, just with a shorter time for the time before handshake completion.

Ideally, the handshake idle timeout should be shorter than the handshake timeout. We ensure this by setting it to max(10s, handshake idle timeout).
This way, we'll be able to give up dialing a peer that's not listening at all earlier. So far, we'd have to wait for the handshake timeout (10s) to finally give up. That's a long time. Now the rule is: If we don't receive any valid QUIC packet for 5 seconds, we give up.

[codecov]

Codecov Report

Merging #2930 (595f6f8) into master (deacefd) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2930      +/-   ##
==========================================
+ Coverage   86.22%   86.22%   +0.01%     
==========================================
  Files         132      132              
  Lines        9141     9145       +4     
==========================================
+ Hits         7881     7885       +4     
  Misses        910      910              
  Partials      350      350              

Impacted Files	Coverage Δ
config.go	100.00% <100.00%> (ø)
session.go	79.47% <100.00%> (+0.07%)	⬆️

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