The HTTP client used in XWikiExtensionRepository is never closed

The general design needs to be changed as it's not really possible to just close it right now because the response sent back by getResponse would be unusable.

It does not seem to cause a noticeable problem in practice, but would be cleaner.

It would also probably make sense to reuse the same client across the ExtensionSession, and take that opportunity to move to HTTP Client 5 (it's currently using HTTP Client 4).