Typedefs to link associated *Requester classes
with their associated "operation"s.
eg. ChannelGet::requester_type is ChannelGetRequester
and ChannelGetRequester::operation_type is ChannelGet.
ChannelFind/List is an ambiguous. All others
are 1-to-1.
ChannelFind::requester_type is ChannelFind
operation_type for both ChannelListRequester and
ChannelFindRequester is ChannelFind.