Fixed internal error on /_store/foo pages.

The issue is caused by MemcacheMiddleware saving infobase store documents as
_store/foo and the same entry is looked up and attempted to use it as thing
when /_store/foo URL is visted.

Added to check to not use memcache when key starts with underscore.
Removed obsolete admin pages.
Log an error instead of failing when search API fails due to input errors.