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

scripts/repository/commit_hook.php

@@ -187,6 +187,11 @@ if (strlen($remote_protocol)) {
   $engine->setRemoteProtocol($remote_protocol);
 }
 
+$request_identifier = getenv(DiffusionCommitHookEngine::ENV_REQUEST);
+if (strlen($request_identifier)) {
+  $engine->setRequestIdentifier($request_identifier);
+}
+
 try {
   $err = $engine->execute();
 } catch (DiffusionCommitHookRejectException $ex) {