Implementation of the `executeQuery` method of `StdCouchDbConnector`,

from the discussion in https://github.com/helun/Ektorp/issues/165,
which prefers `GET` HTTP method even in case of multiple keys.
It is more appropriate for hosted services like <a href="http://cloudant.com/">Cloudant</a>
where `POST` requests are charged more than `GET`.

However, if the HTTP request length exceeds `MAX_KEYS_LENGTH_FOR_GET` characters,
it will use `POST` HTTP method.

`ViewQuery` needed to be factored slightly to support its usage.