Add a RPC call to park a chain

Summary:
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