WIP: Evict error tiles

[maRci002]

Hello, this PR resolves #576 issue, this problem existed even in 0.8.2 release. Fortunately #572 made it possible to support Tile evict calls. TileLayerOptions got a EvictErrorTileStrategy enum which is currently none by default!

Maybe every TileProvider should define a default one if user not provides explicit evictErrorTileStrategy enum, forinstace #564 (-- @bugDim88 anyway what does storage tile caching do with errors? --) may want default none and others not, or MBTilesImageProvider my use none by default because if a Tile not exists on file system then maybe we shouldn't evict Tile image.

0.8.2 preview problem:

0.9.0 preview problem:

Preview the fix with EvictErrorTileStrategy.dispose this behaves like Leaflet if a Tile not retained anymore, and some time later Tile should be refeched this option simulates it:

Preview the fix with EvictErrorTileStrategy.notVisible:

closes #576

[maRci002]

Ok, I think this is a little bit over engineered, maybe I should just listen for connection status (note: it only provides information if there is a network connection, but not if the network is connected to the Internet) and it should redownload the currently visible error tiles too (note: this is not true if TileProvider using filesystem). However it won't fix siituations like this #580.

[maRci002]

Don't merge this PR I'm going to add support for network changes soon.

[johnpryan]

@maRci002 what's the status on this?