Allow new Conduit clients by default.

Summary:
There's no reason to default-reject clients since they can just
pretend to be arc anyway. If they're speaking the right protocol, let them
communicate over Conduit.

Test Plan:
Changed arc to identify as 'arczsdba', ran an arc command.

Reviewed By: simpkins
Reviewers: simpkins
CC: simpkins
Differential Revision: 132