Port recent changes from tcnative

[nmittler]

There have been a few fixes/improvements lately in netty-tcnative. We should port those over to our engine.

A few that immediately stick out:

netty/netty-tcnative#205
(and related netty/netty#6123)

netty/netty-tcnative#207
netty/netty-tcnative#210

netty/netty-tcnative#216
(and related netty/netty#6201)

netty/netty#6118
netty/netty#6113
netty/netty#6090
netty/netty#6089
netty/netty#6439

SSLEngine benchmarks:
netty/netty#6360

[nmittler]

@normanmaurer @Scottmitch @davidben any other PRs that I should absorb from netty land?

[Scottmitch]

netty/netty-tcnative#211. I haven't looked at the new conscrypt code but generally everything that is not cleanup should be interesting and at least worth a look.

[nmittler]

@Scottmitch thanks!

Going forward, It should probably be a goal for conscrypt to make the engine as much of a copy-and-paste from Netty as possible in order to allow us to easily absorb multiple changes all at once.

[Scottmitch]

sgtm ... should make sharing easier in both directions (not sure if you are actively developing conscrypt or just happy to take Netty's improvements).

[nmittler]

We will be actively developing conscrypt, but I expect most of the syncing will be netty->conscrypt at least at the beginning. We also eagerly look forward to contributions from @normanmaurer and @Scottmitch 😜