inbound/outbound handlers

inbound/outbound handlers

Summary:
Much less copypasta this time around. I wonder if the getters and setters for write flags and read buffer settings are necessary in the new handler types, or even if they belong in the bidirectional handler

I'm all ears for more suggestions on reducing copypasta

I'm going to reorg the code (inl headers etc) in a subsequent diff once this is in - easier to review this way

Test Plan: existing unit, thinking about tests for these changes

Reviewed By: davejwatson@fb.com

Subscribers: fugalh, folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2026522

Tasks: 6836580

Signature: t1:2026522:1430346145:bd7f7770eddce0470e2ac72440fc001cf128df08