Support returning time in pipelines

[JeanMertz]

Because now is defined on the MockRedis instance itself, it isn't captured in a "future" when capturing the pipelined block.

Before this change, the added test would return ['bar', 'baz'], instead of ['bar', [1_546_398_246, 0], 'baz'].

Moving now from MockRedis to Database was the simplest way to make this work given how the current futures implementation wraps around a database, which makes it cumbersome / non-idiomatic to capture data returned by the server itself.

I'm open to suggestions how we can make this more generic (i.e. any method on the server itself is captured, and we don't have to move this method to the database layer), without creating too much coupling between the different parts of the system (I don't think we want to start monkey patching the server within a pipelined scope?).