CheckUpdates<true>: verify m_iter->seek_lower_bound(key) failed

[rockeet]

MyTopling MTR test failed with CheckUpdates: verify m_iter->seek_lower_bound(key) failed

[rockeet]

This is caused by MyRocks/RocksDB calling WBWI::Clear() without delete existing iterators.

Fixes: link all WBWI Iterators in with a linked list, and set all iterators invalid in ClearIndex.

I had first add WBWI Iterators in a valvec, then I realized linked list is a more suitable choice.

Relevant commits
131ab66 set all iters invalid on ClearIndex() - use linked list instead of vector
7ba77be set all iters invalid on ClearIndex() - this version use vector(valvec)