@Override
    protected boolean isTileStale(final Tile tile, final TileBitmap bitmap) {
        return this.renderer.getDataTimestamp(tile) > bitmap.getTimestamp();
    }
Whether the tile is stale and should be refreshed.
<br />
This method is called from {@link #draw(org.mapsforge.core.model.BoundingBox, byte, org.mapsforge.core.graphics.Canvas, org.mapsforge.core.model.Point)} to determine whether the tile needs to
be refreshed.
<br />
A tile is considered stale if the timestamp of the layer's {@link #renderer} is more recent than the
{@code bitmap}'s {@link org.mapsforge.core.graphics.TileBitmap#getTimestamp()}.
<br />
When a tile has become stale, the layer will first display the tile referenced by {@code bitmap} and attempt to
obtain a fresh copy in the background. When a fresh copy becomes available, the layer will replace is and update
the cache. If a fresh copy cannot be obtained for whatever reason, the stale tile will continue to be used until
another {@code #draw(BoundingBox, byte, Canvas, Point)} operation requests it again.

@param tile   A tile.
@param bitmap The bitmap for {@code tile} currently held in the layer's cache.
main/src/main/java/cgeo/geocaching/unifiedmap/layers/mbtiles/MBTilesLayer.java