encode to ascii any parameter passed as unicode.

so that whitelisting doesn't have to peek into the data.
added more documentation and some tests stubs too.

my goal here is also to start *real* unit tests for the keymanager, that
do not need an actual soledad implementation. this should make all the
coming tests much faster. I would like to have a common soledad stub
that follows the soledad api, but w/o actual disk persistance, and
migrate all the bulk of the keymanager tests to these unit tests.

This has been previously encapsulated in a dict, and the commands were
not modified accordingly.

I'm adding some very basic test for the KeymanagerService public api
contract.

- Resolves: #8577

in the variant that was merged, we call a failure method to get the
exception, so it's better to test a real failure initialized with an
exception.

besides, I'm starting to hate mocks.

This commit originall contained a fix for a wrong variable name that
avoided the outgoing service to be launched. This had been fixed in a
concurrent mr that got merged before this one. However, this MR
contained a couple of tests for the OutgoingService, and I'm merging
those tests now.