vinyl: use checkpoint_last() instead of vy_scheduler->last_checkpoint

There's no point in maintaining the LSN of the last checkpoint in Vinyl,
we have checkpoint_last() for retrieving it. Make this helper a little
bit more friendly by allowing to omit vclock argument and use it
wherever appropriate.

Here's why I need to do that now. I'm planning to omit calling scheduler
checkpoint functions altogether if there's nothing to dump (so as not to
start worker threads and print 'vinyl checkpoint done' message in case
Vinyl is not used), but one of those functions updates last_checkpoint,
which other pieces of code (gc primarily) relies on. I could factor
last_checkpoint out of vy_scheduler, but why if I can simply use
checkpoint_last() instead.