Improve the send-on-delta policy

[Ravenink]

At the moment, three events can be sent:

1. All data (all variables) from the lab are sent to the client on a periodic basis.
2. A heartbeat with no relevant information might be sent every X seconds.
3. All data (all variables) are sent when the change of a particular variable is bigger than a certain threshold.

There are several improvements to be made:
First, 1) is a subcase of 3).
Second, the value of the threshold (Delta) should be parametrizable both by the lab owner and the client.
Third, right now the variable which change is compared to the threshold is selected by the lab owner in a very ugly way. It should be clean and easy and the client should be able to selected it too.
Fourth, the period for which the Delta condition is checked should also be parametrizable both by the lab owner and the client. The first one is already done, but at the moment this value is set for all experiences defined in the RIP server, but it should be possible to give it different values for each one.