multi()/exec() + pipelining

[melo]

After pull request #10, I'm on the fence wrt the API for multi()/exec() in the presence of pipelining.

The first comments about this were made here:

• Support for pipelining #10 (comment)
• Support for pipelining #10 (comment)
• Support for pipelining #10 (comment)

This issue will track the discussion.

[agimenez]

I just stumbled upon this, is the warning from https://metacpan.org/pod/Redis#transaction-handling-methods valid?

Warning: the behaviour of the TRANSACTIONS commands when combined with pipelining is still under discussion, and you should NOT use them at the same time just now.