Avoid multiple requests for the same voxel subtree

[jjhembd]

Cesium3DTilesVoxelProvider currently checks tile availability at an arbitrary tile coordinate by requesting the subtree containing the availability information for that tile.

A subtree contains availability information for many tiles. It is very likely that Cesium3DTilesVoxelProvider will be loading many tiles from the same subtree simultaneously, so we could have many outstanding requests for the same subtree.

A better approach would likely involve constructing Implicit3DTileContents for subtrees, and deferring any requests for the tiles in those subtrees until the one request for that subtree completes. This would make the approach more similar to non-voxel implicit tilesets.

Using the same approach for all implicit tilesets would also avoid the need for ImplicitSubtree.prototype.tileIsAvailableAtCoordinates and ImplicitSubtree.prototype.childSubtreeIsAvailableAtCoordinates, which are currently only used for voxels.