Cogeo-mosaic

* remove `overview` command (https://github.com/developmentseed/cogeo-mosaic/issues/71#issuecomment-748265645)
* remove `rio-cogeo` dependencies
* update rio-tiler version (`2.0.0rc4`)

* Update to remove all calls to `rio_tiler.mercator` functions.

* update Backend base class for rio-tiler 2.0.0rc3 (add `.feature()` method)

* update for rio-tiler 2.0rc and add backend output models

* raise `MosaicNotFoundError` when mosaic doesn't exists in the DynamoDB table.

* fix typo in DynamoDB backend (https://github.com/developmentseed/cogeo-mosaic/pull/134)
* rename `cogeo_mosaic/backends/http.py` -> `cogeo_mosaic/backends/web.py` to avoid conflicts (author kylebarron, https://github.com/developmentseed/cogeo-mosaic/pull/133)