The name "context" is also used for several other entities. This is
simply confusing. We are getting rid of that fancy word and will leave
it only for context.Context.
Additionally, with the introduction of the new interfaces for most
stuff DeliveryContext struct is no longer used to pass arbitrary
data between modules, it now contains mostly message meta-data and
a few flags, though the latter is now discouraged because it may
lead to problems with concurrency.
Additionally, documentation for some fields in the structure was wrong.
The field DeliveryID is simply renamed to ID for clarity.
Close method does not free the "buffer handle", it discards
the underlying data. It is renamed to Remove.
Interface itself is renamed simply into Buffer since it can be used for
any blob, not just message body.
This allows modules to reference each other during initialization
independently of configuration blocks order.
Also this allows us to initialize default modules in a more clean way.
