Provide an API for integration done so personalities know when they can shutdown

[mhutchinson]

At the moment personalities have no way of knowing whether there is still important background work happening to integrate sequenced entries into the log. For AWS/GCP, this means that a personality could accept a bunch of writes, sequence them, and then shut down. These sequenced entries would not be available via the read API until a storage instance is created again, but the personality has no way of knowing when this work is complete if it didn't keep its own record of the last issued sequence number.

Options:

• Storage has a Shutdown method (prior art: https://pkg.go.dev/net/http#Server.Shutdown).
• We already pass a Context into storage.New - we could cancel this and also provide a Run method or similar in the storage that exits after this context cancellation, but only after integration has happened
• When exiting the personality instantiates a draining storage (lifecycle doc: https://github.com/transparency-dev/trillian-tessera/blob/main/docs/design/lifecycle.md#draining)