box: enable WAL before making initial checkpoint

While a replica is bootstrapped from a remote master, vinyl engine
may need to perform compaction, which means that it may write to
the _vinyl_deferred_delete system space. Compaction proceeds fully
asynchronously, i.e. a write may occur after the join stage is
complete, but before the WAL is initialized, in which case the new
replica will crash. To make sure a race like that won't happen, let's
setup WAL before making the initial checkpoint. The WAL writer is now
initialized right before starting the WAL thread and so we don't need
to split WAL struct into the thread and the writer anymore.

Closes #3968