[RFC] ConnectionPool rewrite and http/2 support

[fabianfett]

This is a draft pr for a ConnectionPool reimplementation that opens the door for http/2 support. This pr has grown very large and I will need to split it up to actually land in parts. However I see benefit in showing the goal down the road. I would love some early feedback. The next step shall be to split code parts out that can go into code review on its own.

Goals

• Reduce the number of pipeline modifications per request (There are no more pipeline modifications necessary)
• Support http/2
• Ease testing by creating explicit state machines.
• No changes in public API at all

A high level overview:

• HTTPConnectionPool is a new connection pool implementation for a single connection target (as in ConnectionPool.Key) replacing HTTP1ConnectionProvider
  • HTTPConnectionPool.StateMachine is the matching StateMachine for the HTTPConnectionPool. It has two potential substate machines:
    • HTTPConnectionPool.HTTP1ConnectionState for a remote targets that support http/1.1 only
    • HTTPConnectionPool.HTTP2ConnectionState for a remote targets that support http/2 only
  • HTTPConnectionPool.Connection is a box around the actual connection for HTTP1Connection and HTTP2Connection that is used in the connection pool state machine
• HTTPConnectionPool.Manager is a new manager of all connection pools. (thus replacing ConnectionPool)
• HTTPConnectionPool.Factory is responsible for creating new HTTP connections and replaces the connection setup currently in Utils.swift
• HTTP1Connection is a wrapper around a Channel. A notable change to before is
• HTTP1ClientChannelHandler is a new channel handler that can be reused for multiple requests.
  • HTTP1ConnectionStateMachine is a new state machine that is used by HTTP1ClientChannelHandler
  • During a request the HTTP1ConnectionStateMachine uses the HTTPRequestStateMachine as a substate machine.

State

• All Logging related tests fail right now.
• http/2 is disabled at the moment.
• http/2 is very under tested
• A few existing tests are commented out, since they rely on implementation details, that are not present in the new implementation.