Support for batch finding multiple objects by id

Change-Id: I2de4ce32caf0e733d51d7f0bfd5914ff6a3695f7
Reviewed-on: http://review.couchbase.org/26542
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>