Use projection option instead of reprojectTo

The `projection` option for vector file sources currently has no good meaning.  The `reprojectTo` option is used to set the projection for cached features.  This change makes it so the projection option is used for this purpose.  The `getProjection` method (not currently exported) can then be used to get the cached feature projection.

This also removes the default of `EPSG:3857` for vector file sources.  If a projection is not configured, features will not be transformed.

Fixes #1631.