Added parked state to getchaintips RPC

Summary: Adding 'parked' state to getchaintips RPC so that it's not confusing when parked chains return 'valid-fork'.

Test Plan: test_runner.py

Reviewers: deadalnix, schancel, Fabien, #bitcoin_abc

Reviewed By: Fabien

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1996