Generic filters/hints for outbox store fetch

[panos-tr]

---- This is WIP at BEST ----

Using the term hint instead of filter to denote that store implementations are not actually required to support these constructs but they are simply provided by the library to allow for improved performance.

interface OutboxHint;

data class InstantProcessingHint(
  val outboxId: Long
): OutboxHint

interface OutboxStore {
  ...
  fun fetch(OutboxHint hint): List<OutboxItem>
}

class OutboxStoreImpl: OutboxStore {
  fun fetch(OutboxHint hint): List<OutboxItem> {
    when(hint) {
      is InstantProcessingHint -> only retrieve item with specified id
      else -> retrieve all items
    }
  }
}