Implement JSON API generation for async API calls

Move JSON helpers to util/rsjson.* for better usability
Implement JSON ostream manipulator to print compact and pretty JSON
Use lambdas for API wrappers, integrate better and avoid namespace
  pollution
Removed experimental JSON API for notify client wrapper, notifications
  can be implemented automatically with moderns async API calls
Implement and automatically expose to JSON API
  RsGxsChannels::turtleSearchRequest(
        const std::string& matchString,
        const std::function<void (const RsGxsGroupSummary&)>& multiCallback,
        std::time_t maxWait )

libretroshare/src/retroshare/rsgxschannels.h

@@ -25,6 +25,7 @@
 #include <inttypes.h>
 #include <string>
 #include <list>
+#include <functional>
 
 #include "retroshare/rstokenservice.h"
 #include "retroshare/rsgxsifacehelper.h"
@@ -266,6 +267,20 @@ public:
 	 */
 	virtual bool ExtraFileRemove(const RsFileHash& hash) = 0;
 
+	/**
+	 * @brief Request remote channels search
+	 * @jsonapi{development}
+	 * @param[in] matchString string to look for in the search
+	 * @param multiCallback function that will be called each time a search
+	 * result is received
+	 * @param[in] maxWait maximum wait time in seconds for search results
+	 * @return false on error, true otherwise
+	 */
+	virtual bool turtleSearchRequest(
+	        const std::string& matchString,
+	        const std::function<void (const RsGxsGroupSummary& result)>& multiCallback,
+	        std::time_t maxWait = 300 ) = 0;
+
 	//////////////////////////////////////////////////////////////////////////////
     ///                     Distant synchronisation methods                    ///
     //////////////////////////////////////////////////////////////////////////////