Add cache access and signal validation wrappers
Add cache access and signal validation wrappers

Use of the main cache structure will grow so added some wrappers
that automate initialization and cleanup.

Refactored duplicate signal handler bus/interface validation
logic into reusable functions.

Updated discovery and signal handlers to make use of all these
new functions.

bradbishop/phosphor-objmgr,bradbishop/phosphor-objmgr,openbmc/phosphor-objmgr,openbmc/phosphor-objmgr,bradbishop/phosphor-objmgr