HttpTxn refactor

Summary:
some refactors to make the code slightly easier to read:
* uses type aliases for Events and State for both Egress and Ingress state machines
* adds a new tryParseContentLength to extract the logic of attempting to parse the content length and LOG(ERROR) if the parsing failed

Reviewed By: afrind

Differential Revision: D102536847

fbshipit-source-id: 77a48c413b687b3a470521ff8cee4de75d19b16c

Author: hanidamlaj