[TLS] Reducing record expansion overhead allowance

[Eric Rescorla <[e-mail]>]

https://github.com/tlswg/tls13-spec/issues/55

In TLS 1.2, we had the following maximum values:

TLSPlaintext: 2^{14}
TLSCompressed: 2^{14} + 1024
TLSCiphertext: 2^{14} + 2048

These overhead values allow for expansion in these transforms
due to potential bad compression overhead or padding, etc.

Wan-Teh Chang points out that we no longer have compression
so there's no need to allow for 1024 bytes of expansion there.

Minimally we should reduce the TLSCiphertext overhead to
2^{14} + 1024. Do people believe that we will have AEAD
ciphers with 1024 bytes of expansion or should we reduce
it further? I'm inclined to not re-judge that and just leave it
at 2^{14} + 1024.

Thoughts?
-Ekr