Add support for retiring old connections for HTTP clients.

[tobz]

Context

When building an HTTP client, we should be able to configure a time-to-live for all pooled connections, such that an individual connection that is idle can never be older than the configured TTL. This is useful for ensuring that new connections are periodically established to the downstream target system, aiding in scenarios like switching over to newly provisioned IPs when load balancer nodes are being replaced, etc.

Notes

This is not something currently exposed by hyper-util but I think, conceptually, could be easy enough to upstream.

[tobz]

Actually, looking at the new additions to hyper-util@v0.1.7, we might be able to use the ported "poison pill" behavior to poison a connection after we get a response and determine the connection is old enough.

Essentially:

• in HttpClient::send, use capture_connection to get Connected for the connection used for the request
• check the response extensions and look for a new type we'll call ConnectionAge (we'll create this as a newtype wrapper around std::time::Instant)
• if we can't find it in the response extensions, we'll create one based on the current time and attach it to the connection via Connected::extra
• if we do find it, we'll see if the connection is older than the configured TTL, and if so, poison the connection via Connected::poison

A poisoned connection won't be returned to the idle pool, so in effect, we're able to mark these connections as no longer usable, and thereby ensuring they get rolled. This is a little lossy, because we're poisoning them right after they get back into the pool, really... and the connection age is attached after the response arrives, so they must end up a few seconds older in reality than what we'd measure if we tracked the actual moment the connection was created.

All of that said, though... it should be more than satisfactory for our purposes.