When immediate rescan is enabled, a poll from Kinesis that returns records and
also indicates we are still behind the latest record will skip waiting for the
next scan interval and rescan immediately.
Related, the initial scan for a shard is performed immediately. The
scan ticker only applies to subsequent scans.
These changes make higher scan intervals feasible, allowing for less
chatty clients while also improving overall effective throughput.
As we work towards introducing consumer groups to the repository we need a more generic name for the persistence layer for storing checkpoints and leases for given shards.
* Rename `checkpoint` to `store`
Major changes:
```go
type ScanFunc func(r *Record) error
```
* Simplify the callback func signature by removing `ScanStatus`
* Leverage context for cancellation
* Add custom error `SkipCheckpoint` for special cases when we don't want to checkpoint
Minor changes:
* Use kinesis package constants for shard iterator types
* Move optional config to new file
See conversation on #75 for more details
