NewSimpleShardTracker creates a new instance of the SimpleShardTracker with the given payment hash and existing attempts.

Shard is a struct used for simple shards where we only need to keep map it to a single hash.

SimpleShardTracker is an implementation of the ShardTracker interface that simply maps attempt IDs to hashes.

PaymentShard is an interface representing a shard tracked by the ShardTracker.

ShardTracker is an interface representing a tracker that keeps track of the inflight shards of a payment, and is able to assign new shards the correct options such as hash and extra records.