Add EvictCallbackFn to handle removing entries from go-memdb when they

are removed from the event buffer.

Wire up event buffer size config, use pointers for structs.Events
instead of copying.

command/agent/testdata/basic.json

@@ -261,7 +261,8 @@
       "data_dir": "/tmp/data",
       "deployment_gc_threshold": "12h",
       "enabled": true,
-      "enable_event_publisher": true,
+      "enable_event_publisher": false,
+      "event_buffer_size": 200,
       "durable_event_count": 100,
       "enabled_schedulers": [
         "test"