introspect: improve info updates

The current _info_update() methods will always reset the change_mask in
the new info structure.

This causes problems if multiple updates are applied to the info before
the rescan in the session manager of pulse-server is excuted. The first
update is cleared and this causes the session manager to sometimes miss
the state changes of nodes and fail to suspend them.

Add a new method to merge with optional reset of the various
introspection info structures. We can use this instead and simply
accumulate all changes until the rescan code has processed all changes.