Implement configurable stats reporting

SYN-287

This requires that HS owners either opt in or out of stats reporting.

When --generate-config is passed, --report-stats must be specified
If an already-generated config is used, and doesn't have the
report_stats key, it is requested to be set.

synapse/storage/registration.py

@@ -289,3 +289,15 @@ class RegistrationStore(SQLBaseStore):
         if ret:
             defer.returnValue(ret['user_id'])
         defer.returnValue(None)
+
+    @defer.inlineCallbacks
+    def count_all_users(self):
+        def _count_users(txn):
+            txn.execute("SELECT COUNT(*) AS users FROM users")
+            rows = self.cursor_to_dict(txn)
+            if rows:
+                return rows[0]["users"]
+            return 0
+
+        ret = yield self.runInteraction("count_users", _count_users)
+        defer.returnValue(ret)