Use solidcache local to the MapnikSource instance

This change moves the `solidCache` to a member of the `MapnikSource` instead of being global.

Benefits:

- Keeps performance boost from caching solid tiles to avoid encoding overhead - 
- Avoids needing to key the cache on the format since the format should be constant for a given `MapnikSource` instance
- Works around stale global cache data for grids that are only keyed on their buffer information and not their attributes - to solve #61/#55 then new `MapnikSource` objects need to be created in interactivity settings are changed.
- Avoid having to move on #63