core: avoid modification of accountSet cache in tx_pool (#21159)

* core: avoid modification of accountSet cache in tx_pool

when runReorg, we may copy the dirtyAccounts' accountSet cache to promoteAddrs
in which accounts will be promoted, however, if we have reset request at the
same time, we may reuse promoteAddrs and modify the cache content which is
against the original intention of accountSet cache. So, we need to make a new
slice here to avoid modify accountSet cache.

* core: fix flatten condition + comment

Co-authored-by: Felix Lange <fjl@twurst.com>