Move compactor and store-gateway to dedicated files

[pracucci]

The jsonnet definitions for compactor and store-gateway are buried into https://github.com/grafana/cortex-jsonnet/blob/master/cortex/tsdb.libsonnet. We should move them to dedicated files, like any other component.

[bboreham]

How is tsdb.libsonnet connected to the rest of this thing?
It doesn't seem to be imported anywhere.

[pstibrany]

We import it manually in clusters that use blocks engine. Clusters that don't use it don't import this file.

[hamishforbes]

We import it manually in clusters that use blocks engine. Clusters that don't use it don't import this file.

This tidbit would be fantastic to have in the readme or even just as a commented out line in the example manifest!
Especially with chunks storage being deprecated and blocks being the preferred default nowadays