MeshUpdateQueue: Add a MapBlock cache that minimizes the amount of MapBlock copying done in the main thread

Cache size is configurable by the meshgen_block_cache_size (default 20 MB).

New profiler stats:
- MeshUpdateQueue MapBlock cache hit %
- MeshUpdateQueue MapBlock cache size kB

Removes one type of stutter that was seen on the client when received MapBlocks
were being handled. (the "MeshMakeData::fill" stutter)

Kind of related to at least #5239

Originally preceded by these commits, now includes them:
- Move the mesh generator thread into src/mesh_generator_thread.{cpp,h}
- mesh_generator_thread.cpp: Update code style
- MeshUpdateThread: Modify interface to house a different implementation: Actual functionality will be changed by next commits.
- MeshMakeData: Add fillBlockData() interface (so that caller can fill in stuff from eg. a MapBlock cache)

builtin/settingtypes.txt

@@ -551,6 +551,11 @@ enable_mesh_cache (Mesh cache) bool false
 #    down the rate of mesh updates, thus reducing jitter on slower clients.
 mesh_generation_interval (Mapblock mesh generation delay) int 0 0 50
 
+#    Size of the MapBlock cache of the mesh generator. Increasing this will
+#    increase the cache hit %, reducing the data being copied from the main
+#    thread, thus reducing jitter.
+meshgen_block_cache_size (Mapblock mesh generator's MapBlock cache size MB) int 20 0 1000
+
 #    Enables minimap.
 enable_minimap (Minimap) bool true