hgweb: establish class for holding per request context Currently, hgweb applications have many instance variables holding mutated state. This is somewhat problematic because multiple threads may race accessing or changing this state. This patch starts a series that will add more thread safety to hgweb applications. It will do this by moving mutated state out of hgweb and into per-request instances of the newly established "requestcontext" class. Our new class currently behaves like a proxy to hgweb instances. This should change once all state is captured in it instead of hgweb. The effectiveness of this proxy is demonstrated by passing instances of it - not hgweb instances/self - to various functions.