This commit adds the input and output attributes to the delivery
resource. These are the maps that are given to, and returned by, the
requester during delivery.

The deliverer service was also refactored in a single package, which is
easier to read. The processor was refactored to be reusable and in
charge of a single purpose, being to merge one template with a message
input, resulting in an interface{} that will be added to the requester
input. The requesters were also refactored to be simple structs.

Some minor changes are also included, including how enclosing {}s were
stringified in tests.

Signed-off-by: Simon Bernier <sbernier@ravnmsg.io>