Fix indexing for chains in different threads (#154) #392

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	6m 37s
test (1.6, ubuntu-latest, x64, 1)	4m 59s
test (1.6, macOS-latest, x64, 1)	6m 35s
test (1.6, windows-latest, x64, 1)	6m 51s
test (1, ubuntu-latest, x86, 1)	6m 36s
test (1, ubuntu-latest, x86, 2)	6m 36s
test (1, ubuntu-latest, x64, 1)	6m 1s
test (1, ubuntu-latest, x64, 2)	5m 55s
test (1, macOS-latest, x64, 1)	7m 3s
test (1, macOS-latest, x64, 2)	6m 39s
test (1, windows-latest, x64, 1)	6m 22s
test (1, windows-latest, x64, 2)	6m 35s
test (nightly, ubuntu-latest, x86, 1)	6m 37s
test (nightly, ubuntu-latest, x86, 2)	6m 35s
test (nightly, ubuntu-latest, x64, 1)	5m 49s
test (nightly, ubuntu-latest, x64, 2)	5m 45s
test (nightly, macOS-latest, x64, 1)	6m 58s
test (nightly, macOS-latest, x64, 2)	6m 41s
test (nightly, windows-latest, x64, 1)	6m 36s
test (nightly, windows-latest, x64, 2)	6m 32s
	2h 8m 22s

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