Better caching

[vincentsarago]

Both obstore Store and Geotiff instance support pickle so we could cache them

[kylebarron]

You shouldn't pickle an obstore store (unless working across processes or something like that). It'll create a new connection pool every time you unpickle. Likewise if you pickle a GeoTIFF, when you unpickle it'll create a new store, which will create a new connection pool.