DerivedSignalBackend needs a get_datakey() method

[canismarko]

Describe the bug

A SignalBackend should have a get_datakey() method that produces a datakey for this signal.

Presently, the metadata are passed in when the signal is initialized, since it inherits from the soft signal backend.

We want this to actually read the metadata from the inherited signals and parse it properly.

Expected behavior

The returned datakey should be determined from the dependent signals.