Add support for multithreaded multipart Glacier uploads.

Part-at-once uploading appears to be capped at a rate of 2 MB/sec on
Amazon's end. This change allows saturation of an outgoing 100M
connection. Uploading parts in parallel is condoned in the Glacier
documentation.
Retry failed part uploads.

A part may fail to upload when, for example, AWS is too busy to handle
an upload request. The delay and retry count should probably be
increased (currently 1 second / 3 times).
Use more reasonable retry semantics and fix bug.

Retry up to 12 times after a delay that doubles each time the request
fails, initially one second.

Bug: Exceptions should be delivered to the control thread as soon as
possible, not whenever sync decides to select the channel get event for
synchronization.
Document changes.