Improve shutdown process

Summary:
Backport of Core PR11006
https://github.com/bitcoin/bitcoin/pull/11006
Prevents a 2-5 second delay every time a node shuts down.

Test Plan:
test_runner.py
Before change:

ALL                                     | ✓ Passed  | 579 s (accumulated)
Runtime: 312 s

After change:

ALL                                     | ✓ Passed  | 325 s (accumulated)
Runtime: 183 s

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, Fabien, #bitcoin_abc

Subscribers: markblundeberg, dagurval, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2544