Introduce query executor strategy, enabling #165

[YannRobert]

This is a refactoring.
This change has no impact on runtime.

This permits to easily change the implementation of StdCouchDbConnector.executeQuery(ViewQuery, ResponseCallback)
It should therefore enable #165 to be fixed in a non intrusive way.

[YannRobert]

TravisCI failed on unit test that has nothing to do with the change

[helun]

LegacyQueryExecutor? How about DefaultQueryExecutor? Names such as LegacyX makes the codebase feel rotten and cumbersome...

[YannRobert]

I agree that Legacy sounds bad.
I was on Default... at the beginning, but had the feeling is would encourage new users to use it whatever new or better alternative may come up in the next ... years.
Any other option you can think of ?

[helun]

It is the default, is it not? And it will always work, although not optimal costwise in the cloudant case.
We could either create a CloudantConnectorFactory that creates a connector configured for cloudant and/or capture this in the docs.

[YannRobert]

Renamed it to DefaultQueryExecutor.

[YannRobert]

rebased from master + squashed class renaming and comments fix

[ceefour]

@helun @YannRobert Thanks :) Now I can wait for 1.4.2 release :)

[ceefour]

@YannRobert @helun 1.4.2 changes are not sufficient to implement my own QueryExecutor properly.

Please review #192 for the additional changes required.