vinyl: don't recover the same run for each its slice

Currently, on recovery we create and load a new vy_run per each slice,
so if there's more than one slice created for a run, we will have the
same run duplicated in memory. To avoid that, maintain the hash of all
runs loaded during recovery of the current index, and look up the run
there when a slice is created instead of creating a new run.

Note, we don't need to do anything like this on initial join, as we
delete the run right after sending it to the replica, so we can just
create a new run each time we make a slice.