update assume valid and chain work
update assume valid and chain work

Summary:
main net defaultAssumeValid corresponds to block height 493942
test net defaultAssumeValid corresponds to block height 1174140

Test Plan: Did make check, and ran binaries

Reviewers: deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D588

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