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/ssh/ssh-exec.php

@@ -8,6 +8,12 @@ require_once $root.'/scripts/__init_script__.php';
 
 $ssh_log = PhabricatorSSHLog::getLog();
 
+$request_identifier = Filesystem::readRandomCharacters(12);
+$ssh_log->setData(
+  array(
+    'Q' => $request_identifier,
+  ));
+
 $args = new PhutilArgumentParser($argv);
 $args->setTagline(pht('execute SSH requests'));
 $args->setSynopsis(<<<EOSYNOPSIS
@@ -248,6 +254,7 @@ try {
   $workflow->setSSHUser($user);
   $workflow->setOriginalArguments($original_argv);
   $workflow->setIsClusterRequest($is_cluster_request);
+  $workflow->setRequestIdentifier($request_identifier);
 
   $sock_stdin = fopen('php://stdin', 'r');
   if (!$sock_stdin) {