Fix missing lock in AvalancheProcessor::getInvsForNextPoll()

Summary:
IsWorthPolling requires cs_main to be held (for
IsBlockFinalized that accesses pIndexFinalized)

Test Plan:

./autogen.sh
mkdir build
cd build
../configure --enable-debug
make -j<your machine number of logic core> check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2296