Allow PhutilChannel to select only write sockets, and expose read buffer status

Summary: Ref T4241. Ref T4206. Small API changes to fix T4241 (next diff). See previous diffs for context.

Test Plan: See next revision.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4206, T4241

Differential Revision: https://secure.phabricator.com/D7775