wal: allow spurious wakeups in wal_write

It's possible to wakeup a fiber, which is waiting for WAL write
completion, using Tarantool C API. This results in an error like:
```
main/118/lua F> Journal result code -1 can't be converted to an error
```

This patch introduces a flag, which is set when WAL write is
finished, that allows fibers to yield until the flag is set.

Closes #6506

NO_DOC=bugfix