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.

CHANGELOG.md

@@ -1,5 +1,12 @@
 # 2018-09-07
 
+## Events cache size increase and configurability for Matrix Synapse
+
+The playbook now lets you configure Matrix Synapse's `event_cache_size` configuration via the `matrix_synapse_event_cache_size` playbook variable.
+
+Previously, this value was hardcoded to `"10K"`. From now on, a more reasonable default of `"100K"` is used.
+
+
 ## Password-peppering support for Matrix Synapse
 
 The playbook now supports enabling password-peppering for increased security in Matrix Synapse via the `matrix_synapse_password_config_pepper` playbook variable. Using a password pepper is disabled by default (just like it used to be before this playbook variable got introduced) and is not to be enabled/disabled after initial setup, as that would invalidate all existing passwords.

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|.."}

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 }}"