Start using std logging

Move towards more powerful and portable logging by initialising and
starting to use the standard logging module in 'arcyd start'.

Log INFO or higher priority messages to 'var/log/info'.

Test Plan:
$ ./testbed/arcyd/test_shell.sh
    .. verify that outputs to stderr file in expected format ..
$ ./precommit.sh
Fix unreliable_git_loop_test.sh

Update bad_git_push_arcyd.py to conform to the new phlsys_git.Repo
interface.

Don't use the supplied arcyd binary when setting up repositories - we
want that bit to succeed.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/unreliable_git_loop_test.sh
    .. run a couple of times ..
    .. make sure it exits cleanly after a few revisions ..
abdt_logging: log using module-level logger

Use the idiomatic module-level logger

Log on_retry_exception's that will be retried again as warnings and
when we have run out of retries then log the last try as an error.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/unreliable_git_loop_test.sh
    .. wait long enough for an attempt to fail ..
    .. stop loop and look at printed /var/log/info ..
Arcyd: show error level name in logging

Make it possible to filter the stream of log messages by severity by
printing the name of the error level in each message.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/unreliable_git_loop_test.sh
    .. wait long enough for a request to fail ..
    .. exit and observe the different log levels ..
abdt_logging: log more detail on_retry_exception

Show more context and the traceback when logging a call to
on_retry_exception().

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/unreliable_git_loop_test.sh
    .. wait long enough for an error to occur ..
    .. stop loop and examine var/log/info for details ..
Arcyd: log errors when reporting exceptions

Update the existing messaging exception handler to also log errors when
reporting.

Test Plan:
Just in case I missed something
$ ./precommit.sh

Exercise the changed code directly with
.. change bad_git_push_arcyd.py to fail all fetches ..
$ ./testbed/arcyd/unreliable_git_loop_test.sh
    .. wait long enough for fetch fail to escalate to handler ..
    .. stop and examine log output to make sure is as expected ..