Root package now contains only initialization code and 'dummy' module.
Each module now got its own package. Module packages are grouped by
their main purpose (storage/, target/, auth/, etc). Shared code is
placed in these "group" packages.
Parser for module references in config is moved into config/module.
Code shared by tests (mock modules, etc) is placed in testutils.
In general, the checks interface with added scoring and quarantining
support was not convenient to use enough. Also it was problematic
to add support for Authentication-Results header field generation.
Per-sender and per-recipient checks were not applied to body.
This is fixed now.
Checks inspecting the message header was able to see header
modifications done by other checks. This could lead to unwanted
side-effects and so now checks can't modify the header directly
and instead can only prepend fields to it by returning them.
Additionally, it allows checks to return values for
Authentication-Results field. Each server handling the message should
add only one field, so it is not possible to implement it using header
prepending.
MsgMetadata.CheckScore is removed, now it is managed internally by
dispatcher code and not exposed where it is not needed.
MsgMetadata.Quarantine is no longer set directly by checks code. Future
refactoring may be remove it altogether as it is discouraged to have
mutable flags in MsgMetadata.
On top of that, tests are added for all new code.
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.
