Waiting cache

A waiting cache is used at the destination of a migration. When you create NewWaitingCache it returns a local and a remote storage.StorageProvider.

• Any reads from the local result in a NeedAt being sent. It will then WAIT until all the data is available.
• Any writes to the remote result in the data being marked as locally available, and will unblock any reads waiting.
• Any writes to the local result in a DontNeedAt being sent, and the block is marked as locally available.
• Reads from remote are unsupported.

It should be noted that only COMPLETE blocks are tracked. For this reason remote WriteAts should be aligned and complete blocks - as from a migrator.