Arcyd: poke_loop - use alice creds for accept

To avoid colliding with the 'phab' user, move the poke_loop test script
to use the built-in 'alice' credentials so that 'poke' and 'arcyd' are
not competing for conduit connections.

N.B. there's a circular queue of 5 conduit connections per user in
Phabricator - if you connect 5 times then any existing connection will
be dropped.

Test Plan: $ ./testbed/arcyd/loop_test.sh
Arcyd: support batch 'snooping' of info/refs

Spend less time doing 'git fetch' by providing a way to specify a
URL for 'info/refs' of each of the repositories.

These URLs can then be fetched in a batch together, re-using
connections where possible.

The contents of the URLs can be watched for changes, when they change
then the corresponding repo must be fetched.

See http://git-scm.com/book/en/Git-Internals-Transfer-Protocols for
more on 'info/refs'.

Add 'phlurl_request' for easy and efficient fetching of URLs from the same
host.

Add 'phlurl_watcher' for easy and efficient montoring of URLs.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
$ ./testbed/arcyd/loop_test.sh

Updated loop_test to also exercise the new snoop url path.