Change HomeServer definition to work with typing.

Duplicating function signatures between server.py and server.pyi is
silly. This commit changes that by changing all `build_*` methods to
`get_*` methods and changing the `_make_dependency_method` to work work
as a descriptor that caches the produced value.

There are some changes in other files that were made to fix the typing
in server.py.

synapse/secrets.py

@@ -25,8 +25,12 @@ import sys
 if sys.version_info[0:2] >= (3, 6):
     import secrets
 
-    def Secrets():
-        return secrets
+    class Secrets:
+        def token_bytes(self, nbytes=32):
+            return secrets.token_bytes(nbytes)
+
+        def token_hex(self, nbytes=32):
+            return secrets.token_hex(nbytes)
 
 
 else: