HTTP2: Handle SETTINGS - master ticket

Handle all settings defined in the spec, ignore all other ones:

- [x] SETTINGS_HEADER_TABLE_SIZE (0x1): https://github.com/akka/akka-http/issues/776
- [ ] SETTINGS_ENABLE_PUSH (0x2): https://github.com/akka/akka-http/issues/775
- [x] SETTINGS_MAX_CONCURRENT_STREAMS (0x3): https://github.com/akka/akka-http/issues/774
- [x] SETTINGS_INITIAL_WINDOW_SIZE (0x4): https://github.com/akka/akka-http/issues/773
- [x] SETTINGS_MAX_FRAME_SIZE (0x5): https://github.com/akka/akka-http/issues/757
- [ ] SETTINGS_MAX_HEADER_LIST_SIZE (0x6): https://github.com/akka/akka-http/issues/778

Also:
- [ ] Handle SETTINGS_TIMEOUT https://github.com/akka/akka-http/issues/777

Start with the HTTP/2 settings listed in this issue and inspect the linked issues 775, 778, and 777 for the remaining work. Confirm the specification requirements for SETTINGS_ENABLE_PUSH, SETTINGS_MAX_HEADER_LIST_SIZE, and SETTINGS_TIMEOUT. Done means all required settings are handled and unknown settings are ignored.

Written by the indexing model from the issue text.