DiskIO should use st_read/st_write to avoid performance issue for ST scheduler.

[winlinvip]

Network IO will use ST's functions, such as st_connect/st_read/st_write, etc. These functions will be scheduled by the ST scheduler and will switch between different coroutines. There will be no scheduling delay or blocking issues.

However, disk reading and writing currently still use the system's open/read/write functions, which actually cause the CPU's time slices to not be scheduled by ST. If the disk IO load is high, blocking phenomena may occur, such as not being able to send data to the client in time.

Currently, disk IO mainly includes:

1. Log.
2. HLS, refer to srs-bench pressure test REUSE PORT multiple processes report "publish timeout, (Interrupted system call)" error #1539.
3. DVR.

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[winlinvip]

We should continue to address this problem.

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[winlinvip]

Disk writing problem, ST cannot solve, must rely on multi-threading, refer to #3308 (comment)

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