Centralized the InstancePerRequest mechanism

Resolves issue #466.
Added constants class to core Autofac for hosting well-known lifetime
scope tags.
Added InstancePerRequest extension to core Autofac that handles
web/API/HTTP requests.
Replaced usage of other lifetime scope tag constants with new central
constant.
Piped other InstancePer***Request methods through the central
InstancePerRequest.
Marked old per-request methods obsolete.
Switched all internal usages of various InstancePer***Request over to
InstancePerRequest.