Refactor how data are stored in browser

[sookoll]

Refactor current solution where under one key there is entire layer group with all layers and all features. This is bad, as one feature change trigger entire layer group rewrite.

Each layer under separate key ('layer/layers/layer_id').
Each feature under separate key ('map/layers/layer_id/feature_id').
Hold array of layer keys in current layers keys ('layer/overlays') to join layers into groups.
Hold array of feature keys in layer.features to join features into layer.