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)

minetest.conf.example

@@ -644,6 +644,12 @@
 #    type: int min: 0 max: 50
 # mesh_generation_interval = 0
 
+#    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.
+#    type: int min: 0 max: 1000
+# meshgen_block_cache_size = 20
+
 #    Enables minimap.
 #    type: bool
 # enable_minimap = true