Core, WebHost: lazy-load worlds in unpickler, WebHost and WebHostLib (#2156)

* Core: lazy-load worlds in unpickler

this should hopefully fix customserver's memory consumption

* WebHost: move imports around to save memory in MP

* MultiServer: prefer loading _speedups without pyximport

This saves ~15MB per MP and speeds up module import if it was built in-place.

* Tests: fix tests for changed WebHost imports

* CustomServer: run GC after setup

* CustomServer: cleanup exception handling

NetUtils.py

@@ -407,14 +407,22 @@ class _LocationStore(dict, typing.MutableMapping[int, typing.Dict[int, typing.Tu
 if typing.TYPE_CHECKING:  # type-check with pure python implementation until we have a typing stub
     LocationStore = _LocationStore
 else:
-    try:
-        import pyximport
-        pyximport.install()
-    except ImportError:
-        pyximport = None
     try:
         from _speedups import LocationStore
+        import _speedups
+        import os.path
+        if os.path.getctime(_speedups.__file__) < os.path.getctime("_speedups.pyx"):
+            warnings.warn(f"{_speedups.__file__} outdated! "
+                          f"Please rebuild with `cythonize -b -i _speedups.pyx` or delete it!")
     except ImportError:
-        warnings.warn("_speedups not available. Falling back to pure python LocationStore. "
-                      "Install a matching C++ compiler for your platform to compile _speedups.")
-        LocationStore = _LocationStore
+        try:
+            import pyximport
+            pyximport.install()
+        except ImportError:
+            pyximport = None
+        try:
+            from _speedups import LocationStore
+        except ImportError:
+            warnings.warn("_speedups not available. Falling back to pure python LocationStore. "
+                          "Install a matching C++ compiler for your platform to compile _speedups.")
+            LocationStore = _LocationStore