Reuse sessions with many origins

[szmarczak]

Fixes #17

TODO:

• [x] Minor bug fixes
• [x] Fully working implementation
• [x] DRY
• [x] Tests
• [x] Update the README
• [x] Fixes #12
• [x] Fixes #22
• [x] RFC comments, so I won't get another headache when reviewing my own code
• [ ] Minor bug fixes

[coveralls]

Pull Request Test Coverage Report for Build 320

• 247 of 248 (99.6%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.8%) to 99.623%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
source/agent.js	166	167	99.4%
<!--	Total:	247	248	99.6%	-->

Totals
Change from base Build 226:	0.8%
Covered Lines:	994
Relevant Lines:	996

---

💛 - Coveralls

[szmarczak]

Blocked by https://github.com/nodejs/node/issues/28985

[szmarczak]

Something's failing: https://travis-ci.org/szmarczak/http2-wrapper/jobs/587629103#L282

[szmarczak]

I'm still working on this. Major refactoring is done here https://github.com/szmarczak/http2-wrapper/tree/origins-2

I'll move to this branch once I'm done refactoring. I'm almost done.

[szmarczak]

Blocked by https://github.com/nodejs/node/issues/29764