Let shuffle always use get_next_message to support holey outboxes

This commit changes the shuffle to always use get_next_message to read
the next outbox message. Before we were using get_message which was only
looking at a specific outbox entry. If this outbox entry was empty, then
the shuffle assumed that the outbox is empty. This did not work if the
outbox contained holes. Now with get_next_message, we always scan until
the next outbox message.

The change itself is trivial. In order to ensure that the change works,
this commit added a few unit tests to ensure the desired behaviour.

Note: With replacing get_message with get_next_message, the shuffle will
perform for every read a RocksDB scan operation. This is highly inefficient
and we should replace this logic with a tailing iterator.

This fixes #1639.

crates/worker/Cargo.toml

@@ -74,6 +74,7 @@ tracing = { workspace = true }
 tracing-opentelemetry = { workspace = true }
 
 [dev-dependencies]
+restate-bifrost = { workspace = true, features = ["test-util"] }
 restate-core = { workspace = true, features = ["test-util"] }
 restate-rocksdb = { workspace = true, features = ["test-util"] }
 restate-schema-api = { workspace = true, features = ["test-util"] }