[Backport 12.1] When session pool is exhausted, binaries servlet can behave incorrectly possibly not freeing locked blocking cache entries

org.hippoecm.hst.servlet.BinariesServlet#getSessionUserID returns null in case of a RepositoryException (and note a NoSessionAvailableException from the session pools is wrapped in a RepositoryException).

After null is returned, org.hippoecm.hst.servlet.BinariesServlet#getPageFromCacheOrLoadPage creates an incorrect cachekey, resulting in the blocking cache being blocked on an incorrect key. If then after this, the

page = getBinaryPage(request, resourcePath);

succeeds, another cachekey is put, resulting in a different key being put (with a non null sessionUserId). The code has grown in this way because many methods are protected, making some changes that would ease the flow of the code impossible because of BC issues.

The fix for now is simple: Let org.hippoecm.hst.servlet.BinariesServlet#getSessionUserID throw an IllegalStateException in case of a repository exception (note that since the method is protected I cannot change its signature without risking to have BC issues for end projects)