Add PoolCluster Feature.

[ifsnow]

It's a feature related to #506

1. support for multiple hosts connection.
2. connection group & retry & selector (round-robin, random, order)

// create
var poolCluster = mysql.createPoolCluster();

poolCluster.add(config); // anonymous group
poolCluster.add('MASTER', masterConfig);
poolCluster.add('SLAVE1', slave1Config);
poolCluster.add('SLAVE2', slave2Config);

// Target Group : ALL(4 group), Selector : round-robin(default)
poolCluster.getConnection(function (err, connection) {});

// Target Group : MASTER, Selector : round-robin
poolCluster.getConnection('MASTER', function (err, connection) {});

// Target Group : SLAVE1-2(wild), Selector : order
// If can't connect to SLAVE1, return SLAVE2. (remove SLAVE1 in the cluster)
poolCluster.on('remove', function (nodeId) {
  console.log('REMOVED NODE : ' + nodeId); // nodeId = SLAVE1 
});

poolCluster.getConnection('SLAVE*', 'ORDER', function (err, connection) {});

// of namespace
poolCluster.of('*').getConnection(function (err, connection) {});

var pool = poolCluster.of('SLAVE*', 'RANDOM')
pool.getConnection(function (err, connection) {});
pool.getConnection(function (err, connection) {});

// destroy
poolCluster.end();

[felixge]

💖 thanks! LGTM / makes sense.

About the test: Not a huge fan of adding an ad-hoc framework / test runner in the test directly. Feels like it's not entirely needed. That being said, if you want to simplify this / or use an existing testing lib (add as devDependency), go ahead.

@dresende @NateLillich any comments?

[ifsnow]

I have deleted an ad-hoc framework. :)

[dresende]

I like the idea, the code looks good. I can test it and improve (if needed) later on. I agree with @felixge about the ad-hoc framework for testing.

[felixge]

LGTM now, feel free to merge unless anybody objects.