net_processing: ignore transactions while in IBD
Summary:
Incoming transactions aren't really relevant until we're caught up. That's why we send a giant feefilter and don't send tx getdatas, but we also shouldn't process them if peers send them anyway. Simply ignore them.
Backport of core#21327.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D16505