community/raft: upgrade to 0.15.0
https://github.com/canonical/raft/releases/tag/v0.15.0
- raft_io version 2 introducing async_work
- raft_fsm version introducing snapshot_async
- disable failing test for libuv 1.44.2 and higher
https://github.com/canonical/raft/releases/tag/v0.15.0
- raft_io version 2 introducing async_work
- raft_fsm version introducing snapshot_async
- disable failing test for libuv 1.44.2 and higher