Add handling for graphing sprocs with the same name but different signature

[kmoppel]

Currently a graph mixing both results is created without a warning

[alexeyklyukin]

A similar, but somewhat different case that is not handled correctly in PGObserver is graphing of several sprocs with the same signature but different schemas. This is especially important when schemas are used to deploy new versions of the sprocs. When multiple versions of the same sprocs are running concurrently the visualization engine tries to connect measurements of one sproc with those of the one from a different schema at the same time point, resulting in vertical lines and a 'straicase' graph.

I think the name culprit is the frontend, namely a query at https://github.com/zalando/PGObserver/blob/master/frontend/src/sprocdata.py (getSingleSprocSQL) that does not distinguish between schemas.

This is not the only change that required, it could be very useful to show versions of the same sproc in different schemas on the same graph, so the UI should be changed as well.