H2 decoder: callback revamp and lots of tests

[graebm]

• Revamped decoder's callback API
  • Goal was to reduce logic required by users (connection & stream)
• Add lots of tests. Each test runs 3 times:
  • Once decoding the input all at once
  • Once decoding the input "one byte at a time". This catches a lot of bugs.
  • Once decoding the input in 2 chunks, split at a specific byte. Re-run the test at every possible split.
    • This didn't turn up any new bugs, not 100% sure it's worth keeping these
• Decoder State Machine changes & fixes:
  • Track the "header-block-in-progress" to make CONTINUATION frames work.
  • Flatten callstack.
    • Some states used to call into each other's function's recursively. Now they just switch the state and let the "state machine" run the next state.
  • Detect any time the payload is too big or small for a given frame type.
  • Lots of little tweaks and bugfixes

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.