If global_factor is omitted, use 0.5 on reload

Consider the following scenario:

1. Start synapse with config that omits a `caches.global_factor`.
2. `CacheProperties.default_factor_size` is now 0.5.
3. Edit config to set `caches.global_factor = 2`.
4. Reload config.
5. Read `CacheProperties.default_factor_size`.
   - Without patch: still 2
   - With patch: backt to 0.5

Without this commit, when we reload config we will compute
`self.global_factor` to be the _previous_ global cache factor. I think
this is surprising: I would that factor to reflect what the config file
says.

synapse/config/cache.py

@@ -180,9 +180,7 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         self.cache_factors: Dict[str, float] = {}
 
         cache_config = config.get("caches") or {}
-        self.global_factor = cache_config.get(
-            "global_factor", properties.default_factor_size
-        )
+        self.global_factor = cache_config.get("global_factor", _DEFAULT_FACTOR_SIZE)
         if not isinstance(self.global_factor, (int, float)):
             raise ConfigError("caches.global_factor must be a number.")