[C++][Dataset] Scanner::ToTable race when ScanTask exit early with an error

[asfimport]

#7180 (comment)

The issue is when Finish exit early due to a ScanTask error, in-flight tasks may try to lock the out-of-scope mutex.

Reporter: Francois Saint-Jacques / @fsaintjacques

Related issues:

• [C++][Dataset] Parquet Dataset factory from a _metadata/_common_metadata file (is related to)

_{Note: This issue was originally created as ARROW-8874. Please see the migration documentation for further details.}