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ARROW-7884: [C++] Relax concurrency rules around GetSize()

RandomAccessFile::GetSize() implementations don't (and shouldn't) move or read the file pointer,
so allow issuing GetSize() calls concurrently.

This should fix a crash (in debug mode only) when reading certain types of malformed
Parquet files.

Closes #6453 from pitrou/ARROW-7884-io-getsize-shared and squashes the following commits:

6bd82f64b6 <Antoine Pitrou> Only call GetSize() once in Parquet file reader
460cfe4f35 <Antoine Pitrou> ARROW-7884:  Relax concurrency rules around GetSize()

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: François Saint-Jacques <fsaintjacques@gmail.com>