http2: refactor Http2Stream and inbound headers

1. Eliminate pooling for Nghttp2Stream instances
2. Use a MaybeStackBuffer for inbound headers
3. Enforce a maximum number of headers per block
4. Enforce a maximum number of header octets per block using MAX_HEADERS_LIST setting

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

http2