Merge #12392: Fix ignoring tx data requests when fPauseSend is set on a peer

Summary:
PR12392 backport https://github.com/bitcoin/bitcoin/pull/12392/files
c4af738 Fix ignoring tx data requests when fPauseSend is set on a peer (Matt Corallo)

Pull request description:

This resolves a bug introduced in
66aa1d58a158991a8014a91335b5bc9c00062f56 where, if when responding
to a series of transaction requests in a getdata we hit the send
buffer limit and set fPauseSend, we will skip one transaction per
call to ProcessGetData.

Bug found by Cory Fields (@theuni).

Probably worth slipping into 0.16 :/.

(cf D2606 - the corresponding diff of ours)

Test Plan:
make check
test_runner.py

Reviewers: deadalnix, jasonbcox, Fabien, #bitcoin_abc

Reviewed By: jasonbcox, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3374