`box.ctl.wal_sync` can be used in lua to wait write flush

[TarantoolBot]

Now one can use box.ctl.wal_sync() to wait until all submitted
writes are successfully flushed to the disk. If write fails it
throws an error. After the function is executed one may reliably
use box.info.vclock for comparisons when choosing a new master

Fixes #10142
Requested by @LevKats in tarantool/tarantool@ea847e1.