Add h2 stream integration tests (swift-server#502)

Always overwrite `Transport-Encoding` and `Content-Length` headers (s…

…wift-server#479)

Only crash in debug mode, if HTTPClient was not shutdown (swift-serve…

…r#478)

### Motivation

Generally we want to inform users that they need to shutdown their HTTPClient. Until `1.6.0` we did this with an assert in HTTPClient's deinit. With `1.6.0` this behavior was raised to a precondition. Because of this adopters might suddenly crash in production where they didn't before.

### Changes

- This pr reverts the current behavior back to something pre `1.6.0`

### Result

- HTTPClient doesn't crash in production anymore.

Support informational response heads (swift-server#469)

Don't crash when hitting long backoffs. (swift-server#458)

Motivation:

If we backoff sufficiently far we can overflow Int64, which will cause
us to crash.

Modifications:

Clamp the backoff value before we convert to Int64.

Results:

No crashes!

Always clear read idle timeout at the end of a request (swift-server#455

)

[HTTPRequestStateMachine] Allow channelReadComplete at any time (swif…

…t-server#450)

[HTTP1Connection] Handle 101 Switching Protocols (swift-server#442)

Add missing availability to APIs requiring macOS 10.15 / iOS 13.0 (sw…

…ift-server#411)

Refactor Channel creation (swift-server#377)

- The connection creation logic has been refactored into a number of smaller methods that can be combined
- Connection creation now has a logical home. It is moved from `Utils.swift` into a `ConnectionFactory`
- There are explicit `ChannelHandlers` that are used for connection creation:
  - `TLSEventsHandler` got its own file and unit tests
  - `HTTP1ProxyConnectHandler` got its own file and unit tests
  - `SOCKSEventsHandler` got its own file and unit tests
- Some small things are already part of this pr that will get their context later. For example: 
  - `HTTPConnectionPool` is added as a namespace to not cause major renames in follow up PRs
  - `HTTPConnectionPool.Connection.ID` and its generator were added now. (This will be used later to identify a connection during its lifetime)
  - the file `HTTPConnectionPool+Manager` was added to give `HTTPConnectionPool.Connection.ID.Generator` already its final destination.