Added Close method to QueryExecutor interface and Mock implementation

[krystsinaetc]

I added Close() method to Mock, because without it it's impossible to use Mock as QueryExecutor interface.

[CMogilko]

This PR is not only adding method to the Mock. It's about adding method Close() to the QueryExecutor interface. That API change may break people's self-written mocks.

[krystsinaetc]

@CMogilko Yes, and this is the problem! It would be great to close session somehow:

type RethinkStore struct {
	session r.QueryExecutor // can't call Close() for *Session
	conf    *RethinkConf
}

[krystsinaetc]

@CMogilko other solution is to create a separate interface which the same as current interface but additionally includes Close method, could it be worth? Otherwise, I will have to create a new interface in my own program as well as Mock structure with gorethink embedded Mock structure and this Close method.

[CMogilko]

This PR is useful for testing, but it changes the API, so it can be accepted in v4.0. And it is need more "smart" mocking of Close().

[gabor-boros]

@CMogilko what did you mean "more smart" mocking exactly?

[CMogilko]

I mean the ability to set error from tests, not only return nil