Raft: decide on worker fiber vs async writes

Initially in Raft implementation it was decided to use ev_check/ev_prepare watchers to dump Raft state on disk when it changes. This didn't work, because these watchers are invoked from a scheduler fiber, where it is not possible to yield.

Need to check if this is true - perhaps it is not possible to yield only in ev_check because it is last. And ev_prepare would work.

Also Raft has another watcher - ev_timer to track when leader dies, and when election times out.
For dumping currently Raft has a worker fiber to perform state dumps, as a workaround.

So in total, Raft uses both fiber and watcher to perform "async" work. That is inconsistent. Either ev_timer should be removed, and the worker fiber should do the waiting, or the fiber should be removed, and the dumping should be done using async WAL writes with a completion callback.

Start in the Raft implementation by verifying whether yielding is possible from ev_check and ev_prepare, then trace the ev_timer, worker fiber, and async WAL write paths. The issue names no files or tests; done means choosing and documenting one consistent approach for waiting and state dumps, with the unused mechanism removed.