SSP-1273 Workaround Liquibase re-init on shutdown

Tries to do this on a number of fronts:

  1) Skip Liquibase init if the current ApplicationContext is
     already inactive. (Unlikely to have any real effect)
  2) Destroy the task scheduler (the underlying executor, actually)
     before any other Spring beans are destroyed
  3) Add interruption-awareness to scheduled jobs so they short-circuit
     in response to #2