[Parse incomplete chunks 5/9] Add feature flag for lazy parsing, integrate into event parser

[benkilimnik]

Summary: Adds vizier feature flag for lazy parsing (off by default) which enables lazy parsing and disables adding filler events to incomplete chunks for supported protocols. Currently only HTTP lazy parsing is supported. Also updates the HTTP parse_test.

Type of change: /kind feature

Test Plan: Tested E2E + unittests added in next PR.