Presence server

[kjnilsson]

This adds a simple presence server on each rabbit node that sends heartbeats to all other configred rabbbit nodes and maintains a local view of which peers are present.

This can be used to get the list of present nodes for functions where it is more important to return a result than to be 100% correct (which is very hard anyway).

Proposed Changes

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• Bug fix (non-breaking change which fixes issue #NNNN)
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Further Comments

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