reduce memory footprint of undecryptable packet handling

[marten-seemann]

1. Don't preallocate a slice for undecryptable packets. We only ever expect to receive undecryptable packets in the case of reordering or packet loss during the handshake.
2. Drop the slice used for undecryptable packets when the handshake completes. We won't queue any undecryptable packets after the handshake (as it's not possible that any undecryptable packet will become decryptable later), so we can hand the slice back to the GC.
3. Assert that we actually don't try to queue undecryptable packets.

[codecov]

Codecov Report

Merging #2932 (c8626d8) into master (deacefd) will decrease coverage by 0.02%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #2932      +/-   ##
==========================================
- Coverage   86.22%   86.20%   -0.02%     
==========================================
  Files         132      132              
  Lines        9141     9143       +2     
==========================================
  Hits         7881     7881              
- Misses        910      911       +1     
- Partials      350      351       +1     

Impacted Files	Coverage Δ
session.go	79.23% <33.33%> (-0.17%)	⬇️

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