Do not increase the banscore of node forwarding transactions that are not valid yet.

Summary: This causes a risk of node banning legitimate peers during inintial sync if that peer relays a lot of transactions. Hopefully the banscore increase is not too high so the practical impact is not huge, but it is still worth fixing.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, awemany, sickpig, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, awemany, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D385