integrate AgentAlertManager services in PlatformAgent

This is now working for the most part, with all platform tests passing (also tested with a couple of the tests in test_instrument_agent).

One visible difference wrt to the code before this integration is that all platform statuses (aggstatus and rollup_status) get to the OK value by the time the platform is in COMMAND mode. Previously, that status was UNKNOWN. Tests adjusted accordingly.

A pending element of the integration is the updates needed upon values received during resource monitoring.