Scale-out storage: read replicas on MySQL, tested (ST-10, ST-15, tests 17 to 19)

Two source-and-replica pairs run in containers: one replicating with
GTIDs, one by binary log position. mysql_replica_tests covers test 17
(tests 9, 10 and 12 with GTIDs) and mysql_replica_position_tests covers
test 18 (lag from Seconds_Behind_Source, and a replica whose account
lacks REPLICATION CLIENT getting no reads) and test 19 (a parallel
replica without replica_preserve_commit_order left out at startup).

The lag reader now takes Seconds_Behind_Source whatever numeric type the
server returns, accepts the older column name, and says in the log why it
gave up measuring.

tests/src/store/mod.rs

@@ -12,6 +12,8 @@ pub mod query;
 pub mod registry;
 #[cfg(feature = "postgres")]
 pub mod replica; // inbuxa: read replicas
+#[cfg(feature = "mysql")]
+pub mod replica_mysql; // inbuxa: read replicas on MySQL
 pub mod scaleout; // inbuxa: scale-out storage
 #[cfg(any(feature = "postgres", feature = "mysql"))]
 pub mod sql_timeout;