Implement message buffer for topics

[nightduck]

Should be independant from rmw interface for encapsulation reasons. Functionality to implement:

• [x] Structure of the table itself, multiple columns for each domain, and an array for metadata (locks, pub/sub counts, etc)
• [x] register_publisher. Takes publisher object and topic name, either creates or resizes topic buffer
• [x] register_subscriber. Like above
• [x] publish. Takes now populated message, looks up/computes offset, and stores it in message queue. Updates flags
• [x] take. Given interested subscriber, fetches next available message, possibly copying it into correct domain
• [x] unregister_publisher. Updates flags, and deletes buffer if last entity. Possibly downsize unused domain? Or keep it?
• [x] unregister_subscriber. Updates flags, and deletes buffer if last entity. Possibly downsize unused domain? Or keep it?
• [x] Unit test 7 functions above

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