Introduce the concept of parked chain.

Summary:
A parked chain behave pretty much like an invalid chain as far as chain selection goes, but having 2 different states will allow to reconsider parked chain in time.

Depends on D1466

Test Plan:
Added test for the flag mechnery itself.

I was unable to find a proper way to test the change in FindMostWorkChain as nothign set the flag yet - so intergration test do not quite work - and the whole thing is so screwed up that unit testing is not really viable.

Reviewers: #bitcoin_abc, schancel, jasonbcox

Reviewed By: #bitcoin_abc, schancel, jasonbcox

Subscribers: jasonbcox, schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1469