MXS-2920: Allow max_slave_replication_lag=0

A requirement of no lag should be allowed. The reason why it wasn't was
simply due to the selection of the default value.

server/modules/routing/readwritesplit/readwritesplit.hh

@@ -124,6 +124,9 @@ static const char gtid_wait_stmt[] =
     "SET @maxscale_secret_variable=(SELECT CASE WHEN %s('%s', %s) = 0 "
     "THEN 1 ELSE (SELECT 1 FROM INFORMATION_SCHEMA.ENGINES) END);";
 
+static const char DEFAULT_MAX_SLAVE_REPLICATION_LAG_STR[] = "2147483647s";
+static const int DEFAULT_MAX_SLAVE_REPLICATION_LAG = 2147483647;
+
 /** Function that returns a "score" for a server to enable comparison.
  *  Smaller numbers are better.
  */