Immutable For Each on ShardStore Checkpoints

[ec2]

The only ways to iterate over Checkpoints in the ShardStore is

1. Use the with_checkpoints method. The problem with this is that it require a mut reference to the store which isn't always available (e.g. in serialization).
2. Get the min_checkpoint_id and the max_checkpoint_id and then casting that to some primitive type if CheckpointId is From/To <numeric> then calling get_checkpoint over the range of that (very not ergonomic!!!).

This PR introduces for_each_checkpoint which is literally the same as with_checkpoints but takes an immutable reference to the store.

I actually wanted to rename with_checkpoints to for_each_checkpoint_mut and call this for_each_checkpoint OR call them with_checkpoints and with_checkpoints_mut but I didn't want to make a breaking API change!

[nuttycom]

utACK 16eff25 for functionality, this can be merged once a CHANGELOG entry has been added.

[ec2]

utACK 16eff25 for functionality, this can be merged once a CHANGELOG entry has been added.

CHANGELOG entry added

[nuttycom]

utACK