Make SmarterCSV thread-safe #444

Job	Run time
test_matrix (ubuntu, 2.5)	23s
test_matrix (ubuntu, 2.6)	22s
test_matrix (ubuntu, 2.7)	27s
test_matrix (ubuntu, 3.0)	10s
test_matrix (ubuntu, 3.1)	13s
test_matrix (ubuntu, 3.2)	9s
test_matrix (ubuntu, 3.3)	10s
test_matrix (ubuntu, head)	15s
test_matrix (ubuntu, truffleruby)	36s
test_matrix (ubuntu, truffleruby-head)	45s
test_matrix (ubuntu, jruby)	42s
test_matrix (ubuntu, jruby-head)	46s
test_matrix (macos, 2.5)	1m 10s
test_matrix (macos, 2.6)	1m 30s
test_matrix (macos, 2.7)	1m 30s
test_matrix (macos, 3.0)	19s
test_matrix (macos, 3.1)	19s
test_matrix (macos, 3.2)	25s
test_matrix (macos, 3.3)	48s
test_matrix (macos, head)	22s
test_matrix (macos, truffleruby)	1m 2s
test_matrix (macos, truffleruby-head)	2m 42s
test_matrix (macos, jruby)	47s
test_matrix (macos, jruby-head)	52s
finish	0s
	16m 44s

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