Durable queues:

This is a disk-backed queue for persisting the messages.
The tasks are served in a FIFO manner.
Durable queues append serialized messages to a fixed-sized file called LogSegment.
The durable queues also append queue operations to an IndexSegment.
The index segment serves as both a :
- Bitcask index for storing the messages offsets
- A WAL file: it is a Log where we append message status after each queue operation (ready, delivered, acked, and failed) and an offset to the message in one of the LogSegments. At init time, we just scan this file to rebuild the in-memory data structure.

Consumers

Implement an acking mechanism for consumers.
Changes the consumer join() to wait for the queue to be empty before canceling the consumers.