Collage: ZeroConf support and node discovery

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Requested by @tribal-tec: Announce node id and connection parameters using servus/ZeroConf, and use this information in LocalNode::connect(id). Expose the servus::Service so applications can add their one key/value pairs for additional information.

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Issue 1: Expose Service or only add set( key, value ) method to LocalNode?
Resolved: Use API as defined below.

Internally we'll use the service to discover() other nodes during connect(). Exposing the full Service risks races if the application does the same thing.

Adding only a set() method limits functionality. It allows to have a stable API, the set() without servus would be an empty implementation.

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Service name will be '_collage._tcp'

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API:

Zeroconf co::LocalNode::getZeroconf();
class Zeroconf
{
    // has a servus::Service and copy of discovered data
   void set( const::std::string& key, const std::string& value );
   Strings getHosts() const;
   Strings getKeys( const std::string& host ) const;
   bool containsKey( const std::string& host, const std::string& key ) const;
   const std::string& get( const std::string& host, const std::string& key ) const;
};