* op-service: add queue package

* batcher: use Queue type for blocks

* revert changes to errors.As/Is

* implement and use Peek operation

* queue: add unit tests

* add godoc

* add more test cases

permute expected / got

* ensure enqueue and prepend are noops when args is empty

* use queue.PeekN and queue.DequeueN

* typo

* queue: simplify method implementations

* revert to old dequeue impl