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/defaults/main.yml

@@ -71,6 +71,9 @@ matrix_synapse_report_stats: false
 # Controls password-peppering for Matrix Synapse. Not to be changed after initial setup.
 matrix_synapse_password_config_pepper: ""
 
+# Controls the number of events that Matrix Synapse caches in memory.
+matrix_synapse_event_cache_size: "100K"
+
 # A list of additional "volumes" to mount in the container.
 # This list gets populated dynamically based on Synapse extensions that have been enabled.
 # Contains definition objects like this: `{"src": "/outside", "dst": "/inside", "options": "rw|ro|slave|.."}