Fix Pull Request Cancelling

[hannahhoward]

Goals

Currently, closing the data transfer on a pull request would always hang or return an error

Implementation

• add tests for channel closing on the responder side (w/ paused channel)
• fix some state machine stuff that led to a statemachine error after request completed
• fix the underlying locking issue (holding lock too long) that led to close call hanging

Sidebar:

• also adds a test to verify ability to send the same data to many people at once -- this was something I wrote earlier but never committed

[ingar]

LGTM