Guard vRecvGetData (now in net processing) with its own mutex

Summary:
This requires slightly reorganizing the logic in GETBLOCKTXN to
maintain locking order.

This is a backport of core#19911 [5/6]
https://github.com/bitcoin/bitcoin/pull/19911/commits/ba951812ec0cc8ebee5911a582f188525b76ff0a

see D10187 for the removal of the !cs_main negative lock

Depends on D10529

Test Plan:

cmake .. -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_BUILD_TYPE=Debug -DENABLE_SANITIZERS=thread
ninja && ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10530