Refactor Http1Session to derive from ProxySession

[a-canary]

A step towards H2-to-Origin.
This contains minimal change to compile, and a few TODO comments. This will be followed by many PRs to refactor and cleanup methods and variables.

[a-canary]

Ready for review @shinrich @masaori335 @maskit @ZhangZizhong

[maskit]

I think the clang-format should be done by a separate PR because it would make backporting complicated.

[maskit]

clang-format was done by #5902. Please remove the commit.

[shinrich]

Is this a functional intermediate step? I assume the original versions of these methods are still around and are being used instead of the new stubs?

[a-canary]

These stubs are the undefined virtual functions of ProxySession. Some existing functions matched the name but not the signature.
Yes, this an intermediate step. Note the "// TODO: refactor this"

[a-canary]

removing format and rebasing

[maskit]

This contains minimal change to compile, and a few TODO comments. This will be followed by many PRs to refactor and cleanup methods and variables.

I'm ok with this change, but I'm a bit worried about how long this journey takes.

• Does / Should the refactoring block 9.0 release?
• Can we release 9.0 in the middle of refactoring? (Will master be in good shape on each step?)

[a-canary]

• Does / Should the refactoring block 9.0 release?
  No. I don't intend to change the TSAPI's so these refactors should continue after 9.0 is released and backported to 9.x

• Can we release 9.0 in the middle of refactoring? (Will master be in good shape on each step?)
  My goal is to keep the PRs small scope and frequent, without breaking anything in each PR. So I'm relying on the CI and unit test a lot to get the changes through. To proceed in this manner without breaking anything, I will take extra steps (like stubbing in functions with TODO), and refactor one dependency at a time.

At the moment I have 4 commits queued up for separate PRs. Because like you said, I want the CI to verify each commit to keep the master branch stable.

[maskit]

No. I don't intend to change the TSAPI's so these refactors should continue after 9.0 is released and backported to 9.x

This is up to RM, but basically backports to 9.0.x are only open for bug fixes once 9.0.0 has been released, 9.1 can have other changes though.

I don't think passing tests is enough. Intermediate state can unnecessarily increase complexity. At the moment, Http1ServerSession doesn't need to extend ProxySession. To people who don't know the plan, this doesn't make sense. IMO, it's fine on feature branches, but not on release branches.

Why don't we have a feature branch for this refactoring?

[a-canary]

Ok. i'm going to close this PR and open merge this into a feature branch called h1outbound.