Fix indexing for chains in different threads #382

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	6m 54s
test (1.6, ubuntu-latest, x64, 1)	5m 3s
test (1.6, macOS-latest, x64, 1)	5m 41s
test (1.6, windows-latest, x64, 1)	15m 3s
test (1, ubuntu-latest, x86, 1)	8m 57s
test (1, ubuntu-latest, x86, 2)	8m 56s
test (1, ubuntu-latest, x64, 1)	6m 3s
test (1, ubuntu-latest, x64, 2)	5m 55s
test (1, macOS-latest, x64, 1)	7m 56s
test (1, macOS-latest, x64, 2)	7m 46s
test (1, windows-latest, x64, 1)	7m 6s
test (1, windows-latest, x64, 2)	7m 14s
test (nightly, ubuntu-latest, x86, 1)	8m 36s
test (nightly, ubuntu-latest, x86, 2)	7m 44s
test (nightly, ubuntu-latest, x64, 1)	5m 53s
test (nightly, ubuntu-latest, x64, 2)	5m 45s
test (nightly, macOS-latest, x64, 1)	6m 14s
test (nightly, macOS-latest, x64, 2)	7m 51s
test (nightly, windows-latest, x64, 1)	7m 8s
test (nightly, windows-latest, x64, 2)	6m 37s
	2h 28m 22s

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