Doc: first pass at release process documentation.
Update STATUS for 5.1.1
Doc: Refinements for release management docs.
[TS-2683]: Additional config enhancements per Leif's suggestions
update docs with the new config format
Fix background_fetch build
TS-3156: remove MutexLock bool operators

  - Mutex[Try]Lock Structs loose lose APIs and bool() operator
  - change structs to classes
  - is_locked() API replaces bool() operator
  - remove unnecessary ink_scoped_mutex and ink_scoped_try_mutex
TS-2729: move HPACK and Huffman code to libhttp2
TS-2729: extend the notion of HPACK field type

Add hpack_parse_field_type() to detect the HPACK field type, and
extend the enumerated type to include all HPACK fields. Remove raw
bit testing where possible.
fix some minor text formatting in the docs for background fetch
Add traffic_via to .gitignore
Add support for Content-Length in background_fetch config
update CHANGES for TS-2683
TS-2683: build fix
TS-2729: add missing error constant
Fix build problem for CentOS, Ubuntu
Tidy up background_fetch includes
Fix release date
TS-3171: minor style updates to Tokenizer

Make some minor style tweaks to Tokenizer so that it is a little
more conventional and easier to use. Rewrite the regression test
so it tests something.
TS-1432: Adding TSMutexDestroy to API