[VL] Fix inaccurate calculation of task slot number used by s.g.s.c.b.v.IOThreads

[zhztheplayer]

COLUMNAR_VELOX_CONNECTOR_IO_THREADS.defaultValueString is not accurate enough when it's used in backend initialization because:

1. SQLConf may not be initialized at that time
2. N in local[N] mode is not counted

The patch fixes the issue.

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[GlutenPerfBot]

===== Performance report for TPCH SF2000 with Velox backend, for reference only ====

query	log/native_6071_time.csv	log/native_master_06_13_2024_142cf0fbc_time.csv	difference	percentage
q1	36.05	35.57	-0.488	98.65%
q2	23.65	23.78	0.133	100.56%
q3	39.23	39.61	0.377	100.96%
q4	32.26	32.76	0.500	101.55%
q5	70.68	67.54	-3.139	95.56%
q6	10.69	9.42	-1.273	88.09%
q7	81.71	80.69	-1.017	98.76%
q8	85.51	84.77	-0.743	99.13%
q9	119.51	120.70	1.190	101.00%
q10	44.04	45.25	1.212	102.75%
q11	20.28	19.79	-0.490	97.58%
q12	24.64	27.92	3.276	113.30%
q13	38.16	39.43	1.275	103.34%
q14	19.89	18.98	-0.910	95.43%
q15	33.36	33.04	-0.313	99.06%
q16	14.11	13.85	-0.264	98.13%
q17	103.72	101.57	-2.149	97.93%
q18	144.71	144.97	0.256	100.18%
q19	15.05	13.79	-1.256	91.65%
q20	26.97	26.68	-0.294	98.91%
q21	256.81	260.83	4.022	101.57%
q22	15.01	13.91	-1.099	92.68%
total	1256.05	1254.85	-1.195	99.90%