PERF: to_epsg causes data loading to be slow for CRS without EPSG code

[snowman2]

This issue is appearing again: #1222

After profiling the load call:

It appears that the rasterio CRS object is being converted to a datacube CRS object about 3750 times in a single load call. The main source of the load time is the call to to_epsg.

I understand that there is a CRS cache to make things faster. I have been thinking about this and I and starting to think that the EPSG form will not be any better than a WKT when you are loading data from the same data source as is done in datacube.

I propose:

1. Remove to_epsg entirely from the cache form and rely only on to_wkt
2. Lazily load to_epsg in the CRS class.

Thoughts?

cc: @alfredoahds