vinyl: count number of slices per run

Currently, there can't be more than one slice per run, but this will
change one the single memory level is introduced. Then we will have to
count the number of slices per each run so as not to unaccount the same
run more than once on each slice deletion. Unfortunately, we can't use
vy_run->refs to count the number of slices created per each run,
because, although vy_run->refs is only incremented per each slice
allocated for the run, this includes slices that were removed from
ranges and stay allocated only because of being pinned by open
iterators. So we add one more counter to vy_run, slice_count, and
introduce new helpers to be used for slice creation/destruction,
vy_run_make_slice() and vy_run_destroy_slice(), which inc/dec the
counter.