chore: rename Multiplex into Reuse in the struct fields

[flaneur2020]

Description

this pr renamed struct field from Multiplex to Reuse in StorageOption and TaskManagerOption.

this is a small renaming in the inner implementation, the field name in the config is still not changed to keep backward compatibility.

Related Issue

Motivation and Context

the term "Multiplex" was a bit misleading in the context of this feature. the functionality actually deals with reusing underlying storage for tasks with the same ID, not multiplexing in the traditional networking sense (in IO or channels).

the term "Reuse" is kinda more accurately describes what the feature does: it enables the system to reuse cached data from completed peer tasks rather than re-downloading them. the existing code already uses the "reuse" term in the implementation, and this pr tries renaming the struct fields to align with the implementation.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)
• Small renaming

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[gaius-qi]

LGTM

[chlins]

lgtm