WL#3793: Use multiple ndb cluster connections in mysqld

We have some performace issues when scaling with mysql clients.

Standard solution is to scale oput with mysqld's instead.  This solution is not
accepted by all customers.  Hence this feature.

So we allow a mysqld to use several connections to the cluster effectively
mimiking beind several mysqld's.

Minor task; no HLS needed. See LLD for details.

- add switch --ndb-cluster-connection-pool  to set how many connections should
be used

- setup all connections at connect, store in array

- allocate connections to thd's in a round robin fashion