New HaMode: sequential write, loadbalance read

This is a proposal / feature request for a new HA mode (sequential write, loadbalance read) in the MariaDb Connector/J.

When running a multi-master cluster (i.e. Galera), writing to more than one node can lead to optimistic locking errors (weirdly called "deadlocks"), see https://galeracluster.com/library/kb/deadlock-found.html.
Writing concurrently to multiple nodes also doesn't bring a whole lot of performance, due to having to (synchronously) replicate to all 3 nodes anyway.

The recommended workaround is to use a read-write splitter (e.g. MaxScale) which would redirect writes to one node, but allow reading from multiple.

Such read-write splitting looks like something that can be easily done client-side by the MariaDB Connector/J, by adding a new HaMode.java enum value that will:

• pick the single write node sequentially (to ensure high-availability), but
• load-balance multiple read nodes

An example implementation could be as simple as:

  SEQUENTIAL_WRITE_LOADBALANCE_READ("sequential-write-load-balance-read") {

    @Override

    public Optional<HostAddress> getAvailableHost(

            List<HostAddress> hostAddresses,

            ConcurrentMap<HostAddress, Long> denyList,

            boolean primary) {

      if (primary) {

        return SEQUENTIAL.getAvailableHost(hostAddresses, denyList, true);

      } else {

        return LOADBALANCE.getAvailableHost(hostAddresses, denyList, false);

      }

    }

  },

Note: the implementation is not final/tested, but is rather meant to gauge interest for a potential PR and as a starting point for a discussion.