vinyl: fix lost key on dump completion

vy_task_dump_complete() creates a slice per each range overlapping with
the newly written run. It uses vy_range_tree_psearch(min_key) to find
the first overlapping range and nsearch(max_key) to find the range
immediately following the last overlapping range. This is incorrect as
nsearch rb tree method returns the element matching the search key if it
is present in the tree. That is, if the max key written to a run turns
out to be equal the beginning of a range, the slice won't be created for
it and it will be silently and persistently lost.

The issue manifests itself as crash in vinyl_iterator_secondary_next(),
when we fail to find the tuple in the primary index corresponding to a
statement found in a secondary index.

Part of #3393