drain transferrer read handle when shutting it down

drain transferrer read handle when shutting it down

Fixes updating git index file after getting an unlocked file when
annex.stalldetection is set.

The transferrer may want to send additional protocol messages when it's
shut down. Closing the read handle prevented it from doing that, and caused
it to crash rather than cleanly shutting down.

Draining the handle without processing the protocol seemed ok to do,
because anything it outputs is going to be some side message displayed
at shutdown. Displaying those once per transferrer process that is running
seems unncessary.

Sponsored-by: Dartmouth College's DANDI project