Introduce Buffer Size Configuration and Warning Logs for Sync Optimization

[YinhaoHu]

close #5492

[CLAassistant]

All committers have signed the CLA.

[zhijian-pro]

@YinhaoHu Thank you very much for your contribution, but we think the following approach would be more appropriate.
The issue lies in our sync download logic where we shouldn't be using chunk.NewOffPage to allocate memory.

juicefs/pkg/sync/download.go

Line 76 in b51019d

p := chunk.NewOffPage(int(size))

In this line of code

juicefs/pkg/sync/sync.go

Line 362 in b51019d

downer := newParallelDownloader(src, key, size, 10<<20, concurrent)

we have hard-coded a block size of 10M, so we can directly use a fixed-size sync.pool to allocate memory.