Generate a random unique "Request ID" for SSH requests so processes can coordinate better

Summary:
Depends on D19247. Ref T13109. When we receive an SSH request, generate a random unique ID for the request. Then thread it down through the process tree.

The immediate goal is to let the ssh-exec process coordinate with commit-hook process and log information about read and write lock wait times. Today, there's no way for ssh-exec to interact with the PushEvent, but this is the most helpful place to store this data for users.

Test Plan: Made pushes, saw the PushEvent table populate with a random request ID. Exported data and saw the ID preserved in the export.

Maniphest Tasks: T13109

Differential Revision: https://secure.phabricator.com/D19249