[devtools] Use a trap to cleanup bitcoind instead of a background process

Summary:
This improves code locality, making it easier to reason about.
This also removes an unnecessary looping sleep.

Test Plan:

./bitcoind-exit-on-log.sh --grep 'New outbound peer connected' --params "-datadir=/home/jasonbcox/.bitcoin"
ps aux | grep bitcoind

Verify no dangling processes

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6378