Use PooledContextFactory

[benaadams]

Also needs aspnet/HttpAbstractions#501 to work

Took out SupportsSendFile as it doesn't exist anymore

/cc @DamianEdwards

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[benaadams]

Usual stuff, 4k requests, 16 pipelined, using FF

Pre-allocations for DefaultContext 1.12MB

Post-allocations for DefaultContext 160KB

Almost all of which is logging scope

Not quite sure what's going on with ThreadStatic, but don't use it much - take the locks out and it goes a bit weird.

[AspNetSmurfLab]

Mixed results with this. For non-pipelined tests it seems to improve things, but for pipelined tests it makes this much worse (~10% drop in RPS).

[rynowak]

Sounds like contention yo

[benaadams]

Rookie mistake - didn't actually make the ThreadStatic static

[DamianEdwards]

@benaadams would you mind rebasing this so we can re-measure pls?

[benaadams]

O_o did I rebase on wrong thing?

[benaadams]

Submitted new PR #49