/vsicurl_streaming/ : move the caching of the first megabyte of a file at the file handle level, and not the filesystem level, to avoid inconsistencies when the remote file changes (#5007)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25693 f0d54148-0727-0410-94bb-9a71ac55c965