Summary:
I should've made them structs from the beginning, but of course I thought that
I know better and these tokens can definitely not be richer than just strings.
Well, it turns out we need them to be richer. Specific reason is that in theory
a single Mononoke (or scs_server) instance can run with multiple storage
configs. For us this means that one target's requests may be stored in one
db, while another target's requests - in another one. For blobstores this is
even used in practice, while for xdb it's just a theoretical thing, but we need
to support it nevertheless.
To do so, let's add the ability to query the target (and, correspondingly, the
Mononoke repo) from any king of params our async methods receive: ThriftParams
or Token implementors.
In addition, this diff really implements `AddScubaParams` and `AddScubaResponse` for more things than before, so there's that.
Finally, apart from making tokens structured, this diff also changes an interface in two more ways:
- it adds optional `message` fields to several params structs
- it adds `changesets_to_merge` to `MegarepoChangeTargetConfigParams`
Reviewed By: StanislavGlebik
Differential Revision: D28333999
fbshipit-source-id: 99bd19b040b59ee788ef661dda3171cc56254d33
