ssh: rewrite proxy management for multiprocessing usage

We changed sync to use multiprocessing for parallel work.  This broke
the ssh proxy code as it's all based on threads.  Rewrite the logic to
be multiprocessing safe.

Now instead of the module acting as a stateful object, callers have to
instantiate a new ProxyManager class that holds all the state, an pass
that down to any users.

Bug: https://crbug.com/gerrit/12389
Change-Id: I4b1af116f7306b91e825d3c56fb4274c9b033562
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305486
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>
diff --git a/project.py b/project.py
index 3755806..2f83d79 100644
--- a/project.py
+++ b/project.py
@@ -2045,8 +2045,8 @@
       name = self.remote.name
 
     remote = self.GetRemote(name)
-    if not remote.PreConnectFetch():
-      ssh_proxy = False
+    if not remote.PreConnectFetch(ssh_proxy):
+      ssh_proxy = None
 
     if initial:
       if alt_dir and 'objects' == os.path.basename(alt_dir):