feat: make uuids truly unique

Up to this patch `instance_uuid` and `replicaset_uuid` were generated
using name-based UUID v3. So all instances named `i1` in all clusters
had the same UUID.

With this patch UUIDs become random v4. Particular UUID values are
generated in two locations:

1. `start_boot` (src/lib.rs)
2. `handle_join_request_and_wait` -> `build_instance` (src/rpc/join.rs)

Function `Instance::new()` is removed in favor of explicit struct
construction.