Allow response aggregation for multi-keyed operations in asynchronous mode

Mult-keyed operations behaves a bit different in async mode, they are yielding responses as far as they received. This is good, but it would be nice to have switch to force library to aggregate all of them.

Example:

Couchbase.bucket.run do |c|
c.get("foo", "bar", "baz", :aggregate => true) do |results|
results.each do |res|

1. ...
   end
   end
   end