Arcyd: add script to launch a 'test shell'

Add 'testbed/arcyd/test_shell.sh' for launching a shell in which you can
interact with a new instance of Arcyd.  The instance is cleaned up upon
exiting the shell.

Test Plan:
$ ./testbed/arcyd/test_shell.sh
confirm arcyd process is running with 'ps'
exit, confirm process has gone away with 'ps'
inspect reported savemail content
Arcyd: re-use conduits with same creation params

To greatly reduce the amount of time we spend communicating with
Conduit, re-use conduits between cycles and repositories where they have
the same creation parameters.

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