Add the ability to control event_cache_size for Synapse

I've found the previous 10K default value to be way too low
on a bunch of servers I'm running, so it's now up to
100K and made configurable.

roles/matrix-server/templates/synapse/homeserver.yaml.j2

@@ -186,7 +186,7 @@ database:
     cp_max: 10
 
 # Number of events to cache in memory.
-event_cache_size: "10K"
+event_cache_size: "{{ matrix_synapse_event_cache_size }}"