Stop direct VStream at target VGTID

[nickvanw]

Summary

• stop direct VStream reads as soon as the requested target VGTID is reached
• keep the early-stop path disabled for historical-copy/copy-resume cursors
• add regression coverage that fails if sync waits for a later VGTID after receiving the target commit VGTID

Tests

• env GOCACHE=/tmp/go-build go test ./lib
• env GOCACHE=/tmp/go-build go test ./...
• env GOCACHE=/tmp/go-build go test ./lib -run 'TestSync_DirectVStreamStopsAtTargetVGTIDWithoutWaitingForLaterEvent|TestRead_CanStopAtWellKnownCursor|TestSync_CheckpointsHistoricalCopyProgress|TestSync_DoesNotPeriodicallyCheckpointVGTIDProgress|TestRead_ResumesHistoricalCopyEvenWhenPeekMatchesCopyCursorPosition|TestRead_DoesNotStopAfterProgressingTimeoutWindows' -count=1