Optimize handshake memory usage

[noboruma]

Description

Avoid unnecessary copy of packet header data when pushing to cache.
Avoid unnecessary copy & reallocation inside message certificate marshalling.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.54%. Comparing base (956d41d) to head (6890ea7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #815      +/-   ##
==========================================
+ Coverage   82.53%   82.54%   +0.01%     
==========================================
  Files         120      120              
  Lines        6802     6807       +5     
==========================================
+ Hits         5614     5619       +5     
  Misses        778      778              
  Partials      410      410              

Flag	Coverage Δ
go	82.54% <100.00%> (+0.01%)	⬆️

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[theodorsm]

Thanks! LGTM. Since we don't export handshakeCache, removing the copying in push should be fine.