Add API for poisoning connections

[rcoh]

This is a port of https://github.com/hyperium/hyper/pull/3145/files + a test.

It introduces a PoisonPill atomic onto connection info. When set to true, this prevents the connection from being returned to the pool.

[seanmonstar]

It at least looks like the failed test is relevant. Dropping a client should close its idle connections.

[rcoh]

Yeah I agree. I tried pretty hard to reproduce it on a number of different environments but I haven't been able yet. I'll transition this to Draft in the interim

[Velfi]

I tried running these tests in Docker and in my dev desktop but couldn't reproduce. :(

Running clippy on the tests shows that we're holding a std::sync::Mutex across await points in a few places. Could that be related to the test failure?

I also see that read is used in a few tests but we're not checking to see if it read the whole thing. Switching to read_exact causes a bunch of new test failures. (read 1: Os { code: 35, kind: WouldBlock, message: "Resource temporarily unavailable" })

[seanmonstar]

I just retried CI, and it's red this time too.