Seperated enum class BlockValidity from chain.h into its own .h file and updated other .h and .cpp dependencies.

Summary: Continues T380, dependent on D1655. Created new blockvalidity.h file removing enum class BlockValidity from chain.h. Also updated other .h and .cpp files using BlockValidity.

Test Plan: make check

Reviewers: jasonbcox, deadalnix, schancel, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1656