Sometimes it's more efficient to run a group of tasks all at once rather than
one at a time. With luigi, it's difficult to take advantage of this because your
batch size will also be the minimum granularity you're able to compute. So if
you have a job that runs hourly, you can't combine their computation when many
of them get backlogged. When you have a task that runs daily, you can't get
hourly runs.

For the user, batching is accomplished by setting batch_method in the parameters
that you wish to batch. This takes a list of parameter values (already parsed)
and combines them into a single parameter. You can limit the number of tasks
allowed to run simultaneously in a single batch by setting the class variable
batch_size. If you want to exempt a specific task from being part of a batch,
simply set its is_batchable property to False.

Unlike spotify#1538, this does batch combining on the worker side, allowing arbitrary
batch methods and keeping better track of task ids for things like execution
summary.