validation: Replace MinBIP9WarningHeight with MinBIP9WarningStartTime
requested to merge github/fork/dimitaracev/validation-replace-min-bip9-height-to-min-bip9-start-time into master
This PR addresses comment. Replaces CChainParams::MinBIP9WarningHeight with CChainParams::MinBIP9WarningStartTime. It is then used in WarningBitsConditionChecker::BeginTime to only check block headers that are newer.
This helps reduce the block headers that are being scanned for warnings.
cc @ajtowns