When re-reading all of the mempool code in master I noticed that in 2 places, a working `setEntries` (which is an alias for a
`std::set<CTxMemPool::txiter>`) was being erased-from inefficiently.

In these two places we are grabbing the first `txiter` from the set then we are erasing it.  In the extant code, the `txiter` is being passed as a *key* to the set. Instead, we should be erasing using the slightly more efficient `std::set(std::set::iterator)` overload, which does not need to do any O(log N) lookups.

ninja all check-all