vinyl: set box.cfg.vinyl_write_threads to 4 by default

Any LSM-based database design implies high level of write amplification
so there should be more compaction threads than dump threads. With the
default value of 2 for box.cfg.vinyl_write_threads, which we have now,
we start only one compaction thread. Let's increase the default up to 4
so that there are three compaction threads started by default, because
it fits better LSM-based design.