fix(chain): Consider conflicting ancestors in...

fix(chain): Consider conflicting ancestors in...
...try_get_chain_pos

In try_get_chain_pos, when we notice that a transaction is not included
in the best chain, we check the transactions in mempool to find
conflicting ones, and decide based on that if our transaction is still
in mempool or has been dropped.
This commit adds a check for transactions conflicting with the
unconfirmed ancestors of our tx.

Co-authored-by: Wei Chen <wzc110@gmail.com>