Add .query to PoolNamespace.

[alsotang]

Refactor PoolNamespace#_getClusterNode but still backward-compatible

original issue: #1505

[alsotang]

now we can use cluster this way:

var pool = cluster.of('SLAVE*', 'ORDER');

pool.query('SELECT 1', function (err, rows) {}) // works fine!!

[alsotang]

travis ci fails, but seems a old test result in that.

[dougwilson]

Nice, thanks, @alsotang! Don't worry about that failing test; I'll get it fixed today :)

[dougwilson]

Sorry for the long delay, @alsotang ! I was able to fix the flaky tests, and so the PR is now passing the tests. I do see that the PR is marked failing as well because it caused the coverage percentage to decrease. Looking at the report, it looks like it's because in the new PoolNamespace.prototype.query, both of the if (err) code paths are not tested.

[dougwilson]

Pending tests for new code paths. The only missing ones are displayed at https://coveralls.io/builds/7620472/source?filename=lib%2FPoolNamespace.js

[dougwilson]

lol @alsotang I just popped into this PR to see if I could just add the tests & merge to help out :) I see you just put more more commits. Please ping me when you're ready for me to take a look :)

[alsotang]

I already add tests for the error handler. Seems now the PoolNamespace.js is 100% cover.