Arcyd: save snoop cache once per cycle only

Avoid consistency concerns when saving the git snoop url cache by
hoisting it out of the multithreaded section.

Note that other usage of the cache is thread-safe as consistency only
has to be preserved per-URL. This is ensured because we'll only update a
particular repo on one thread at a time.

Remove the snoop cache sharing concern from the list of multi thread
concerns.

Test Plan:
$ ./precommit.sh
Arycd: extract _ConduitManager for threads to use

Make it possible to share conduits between threads nicely by introducing
a _ConduitManager that can safely allocate new conduits as required.

Add more detail to the conduit multi-threading concerns.

Test Plan:
$ ./precommit.sh