[opt](var) change merge_io_read_slice_size to merge_io_read_slice_size_bytes

[morningman]

No description provided.

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[morningman]

run buildall

[doris-robot]

ClickBench: Total hot run time: 28.01 s

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/clickbench-tools
ClickBench test result on commit 55bea6a69ec63c1ae0fe2a5dc43cc05d3a11f5bf, data reload: false

query1	0.06	0.06	0.05
query2	0.10	0.06	0.05
query3	0.25	0.08	0.09
query4	1.62	0.12	0.12
query5	0.28	0.26	0.25
query6	1.21	0.65	0.66
query7	0.03	0.02	0.03
query8	0.06	0.05	0.05
query9	0.62	0.54	0.53
query10	0.60	0.59	0.59
query11	0.16	0.12	0.12
query12	0.16	0.13	0.13
query13	0.63	0.61	0.60
query14	1.02	1.03	1.01
query15	0.86	0.84	0.86
query16	0.41	0.41	0.43
query17	1.07	1.08	1.07
query18	0.23	0.21	0.21
query19	1.99	1.86	1.84
query20	0.02	0.02	0.02
query21	15.42	0.19	0.14
query22	5.01	0.08	0.06
query23	15.66	0.28	0.10
query24	2.64	0.70	0.46
query25	0.07	0.07	0.06
query26	0.14	0.15	0.14
query27	0.07	0.06	0.06
query28	4.44	1.17	0.95
query29	12.61	4.15	3.38
query30	0.31	0.14	0.11
query31	2.83	0.61	0.39
query32	3.24	0.56	0.46
query33	3.05	3.14	3.07
query34	15.90	5.23	4.54
query35	4.58	4.58	4.63
query36	0.71	0.52	0.49
query37	0.12	0.07	0.07
query38	0.06	0.04	0.05
query39	0.04	0.03	0.03
query40	0.17	0.14	0.14
query41	0.09	0.04	0.03
query42	0.04	0.03	0.03
query43	0.05	0.04	0.04
Total cold run time: 98.63 s
Total hot run time: 28.01 s

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.