Provide BatchingProcessor as a built-in processor

[ocadaruma]

• Task batching is a common-pattern that many Decaton users often implement by their own
  • i.e. Batching several tasks of type T to List<T> and process them at once. e.g. when downstream-DB supports batching I/O (which often very efficient)
  • Batch-flushing should be done in time-based and size-based.
• So it's better to provide BatchingProcessor built-in to meet the common needs

[ryamagishi]

I'll try this issue.

[ocadaruma]

Thank you!
I assigned you.

[ocadaruma]

fixed by #139, #150