unified tile cache

[adam-ce]

Currently, we have one cache per tile layer. This has the following problems

• we will not kick out tiles of unused layers
• inflexible resources allocation among layers, e.g. you have 16k total, but layer x is not used, so allocate 8k to y and 8k to z instead of 5k each.
• storing the memory cache to disk creates more files (slower).

Unified tile cache:

• tile cache has hashmap tile_type -> tile_data per tile. not all slots need to be taken.
• query / traversal functions that only work for a certain tile type (don't descent if it is unavailable)
• tile_type should probably be an int; the int should be chosen by registering a type via string. the registry must be stored to disk.
• the tile cache needs synchronisation, because we likely want to use it from different threads.

[adam-ce]

Switch the tile querier to this unified tile cache, then this part should be good.