vinyl: store run slices in metadata log

In order to recover run slices, we need to store info about them in the
metadata log, so this patch introduces two new records:
 - VY_LOG_INSERT_SLICE: takes IDs of the slice, the range to insert the
   slice into, and the run the slice is for. Also, it takes the slice
   boundaries as after coalescing two ranges a slice inserted into the
   resulting range may be narrower than the range.
 - VY_LOG_DELETE_SLICE: takes ID of the slice to delete.

Also, it renames VY_LOG_INSERT_RUN and VY_LOG_DELETE_RUN to
VY_LOG_CREATE_RUN and VY_LOG_DROP_RUN.

Note, we don't need to keep deleted ranges (and slices) in the log until
the garbage collection wipes them away any more, because they are not
needed by deleted run records, which garbage collection targets at.