[net processing] Move RelayTransaction into PeerManager
Summary:
It moves the RelayTransaction() function to be a member function of the PeerManager class. This is required in order to move the transaction inventory data into the Peer object, since Peer objects are only accessible from within PeerManager.
Backport of core#21162.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D12204