fix: release RefEntry in SkipList when dropping it

[ekexium]

I ran into a memory leak problem when using the SkipMap. Please take a look if this is the root cause.
This probably relates to #1178

The comment of RefEntry says:

/// You *must* call `release` to free this type, otherwise the node will be
/// leaked. This is because releasing the entry requires a `Guard`.

But when RefEntry is overwritten by clone_from, there is no decrement or release.

I can reproduce a memory leak in the test stress_skipmap_range_iter in tikv/tikv#19284