Add a RPC call to park a chain
As per title. This allows to finally test the parking feature a bit more.
Depends on D1950
Test Plan: Added an integration test to ensure RPC call behave as expected.
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: jasonbcox, teamcity
Differential Revision: https://reviews.bitcoinabc.org/D1952