Skip temp files for repo updates

[HebaruSan]

Problem

Windows users have reported "access denied" errors with %TEMP%\CdFileMgr during repo updates, see #3963:

Cause

Repo updates are downloaded to temp files with paths in that folder, which are then loaded back from disk. Some users have found that their anti-virus software was stopping CKAN from accessing these temp files.

Changes

Now repo updates are downloaded into a MemoryStream and processed directly (instead of being saved to disk, then immediately re-loaded and parsed). Since they are around 30 MiB for the biggest repo, this should work fine for everyone, including whomever or whatever is preventing us from reading the temp files.

• NetAsyncDownloader.DownloadTarget is now an abstract base class with child classes:
  • NetAsyncDownloader.DownloadTargetFile - downloads to a file on disk, replaces previous usages of DownloadTarget, and its default temp file path is now a standard path instead of one in the CdFileMgr folder
  • NetAsyncDownloader.DownloadTargetStream - downloads to a MemoryStream, now used by RepositoryDataManager.Update
• Various layers of code are updated to no longer handle certain streams with using () {}, since this closes the MemoryStream and makes it inaccessible for processing after download
• NetAsyncDownloader.DownloadPart.path was redundant with NetAsyncDownloader.DownloadTarget.filename and is now eliminated