Fixes#3020.
In order to write integration tests for some features, especially related to rate limiting, rechecking of CAA, and expiration of authzs, orders, and certs, we need to be able to fake the passage of time in integration tests.
To do so, this change switches out all clock.Default() instances for cmd.Clock(), which can be set manually with the FAKECLOCK environment variable. integration-test.py now starts up all servers once before the main body of tests, with FAKECLOCK set to a date 70 days ago, and does some initial setup for a new integration test case. That test case tries to fetch a 70-day-old authz URL, and expects it to 404.
In order to make this work, I also had to change a number of our test binaries to shut down cleanly in response to SIGTERM. Without that change, stopping the servers between the setup phase and the main tests caused startservers.check() to fail, because some processes exited with nonzero status.
Note: This is an initial stab at things, to prove out the technique. Long-term, I think we will want to use an idiom where test cases are classes that have a number of optional setup phases that may be run at e.g. 70 days prior and 5 days prior. This could help us avoid a proliferation of global state as we add more time-dependent test cases.
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