wal: remove files needed for recovery from backup checkpoints on ENOSPC

Tarantool always keeps box.cfg.checkpoint_count latest checkpoints. It
also never deletes WAL files needed for recovery from any of them for
the sake of redundancy, even if it gets ENOSPC while trying to write to
WAL. This patch changes that behavior: now the WAL thread is allowed to
delete backup WAL files in case of emergency ENOSPC - after all it's
better than stopping operation.

Closes #3822