Implement Liveness, Readiness, and Startup Probes for SDP services

Probes allow the platform to check whether components are responsive, and therefore detect both successful start-up as well as degradation events. Combined with robust component restart (see SP-1884) this should enable the SDP sub-system to self-heal in a number of failure scenarios.

• Define nature of probe (i.e. interface to check) for every part of the sub-system, weighting false-negative (service unresponsive, probe succeeds) against false-positive (service responsive, probe fails). Options might include, but not be limited to:
  1. Simple "does process exist"
  2. Check that a lease key exists in the configuration database (leases need constant keep-alive messages from the creator, or get deleted)
  3. Check that Tango devices exist, attributes are readable and/or a "ping" command gets accepted
  4. When in doubt, can also add a simple local HTTP server to component just for being able to answer probes
• Implement probes as designed
• Stretch: Integration test that demonstrates that a probe on a critical component (Tango interface? That's where we had a deadlock at least once!) works