Implement `readonly` flag in MySQL data store.

[mol123]

In the MySQL datastore, the handling of the readonly flag should be implemented in runInTx.

At least, recent versions of the golang sql package provide a way to pass it in to BeginTx. We don't know how complete the actual mysql driver support is.

[bgalehouse]

I looked into it a bit more, and recent versions of the mysql driver do pass the constraint down, and the InnoDB engine claims to take advantage of it.