[PATCH] CacheManager subscription to IPC

This migrates Cache Manager to using the new IPC subscription API for
handling message events.

To do this the CacheManager is converted to an AsyncJob which runs from
the point of first action registration until shutdown. The existing
event handlers are move into its scope as-is.

Requires the IPC subscriptions patch previously submitted.

FUTURE WORK:
   Migrate the SNMP messaging as well. I'm not sure if we want to do
that before, after or as part of merging the SNMP and mgr backend
message actions.

Amos

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