Disconnect peers violating blocks-only mode

Summary:
If we set fRelay=false in our VERSION message, and a peer sends an INV or TX
message anyway, disconnect. Since we use fRelay=false to minimize bandwidth,
we should not tolerate remaining connected to a peer violating the protocol.

This is a partial backport of Core PR15759 : https://github.com/bitcoin/bitcoin/pull/15759/commits/0ba08020c9791f7caf5986ad6490c16a2b66cd83

Depends on D6424

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6435