Use AphrontRequestStream to read request input

Summary:
Ref T10604. This uses the new standalone stream reader introduced in D15483 to read request data, instead of putting the logic in PhabricatorStartup.

It also doesn't read request data until it specifically needs to. This supports, e.g., streaming Git LFS PUT requests, and streaming more types of requests in the future.

Test Plan: See D15483. Made various different types of requests and wasn't immediately able to break anything.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10604

Differential Revision: https://secure.phabricator.com/D15484