New iteration of the ActivityPub delivery implementation and interface.
Advantages over previous interface:
* When sending a ByteString body, the sender is explicitly passed as a
parameter instead of JSON-parsing it out of the ByteString
* Clear 3 operations provided: Send, Resend and Forward
* Support for per-actor keys
* Actor-type-specific functions (e.g. deliverRemoteDB_D) removed
* Only the most high-level API is exposed to Activity handler code, making
handler code more concise and clear
Also added in this patch:
* Foundation for per-actor key support
* 1 key per actor allowed in DB
* Disabled C2S and S2S handlers now un-exported for clarity
* Audience and capability parsing automatically done for all C2S handlers
* Audience and activity composition automatically done for Vervis.Client
builder functions
Caveats:
* Actor documents still don't link to their per-actor keys; that should be the
last piece to complete per-actor key support
* No moderation and anti-spam tools yet
* Delivery API doesn't yet have good integration of persistence layer, e.g.
activity is separately encoded into bytestring for DB and for HTTP; this will
be improved in the next iteration
* Periodic delivery now done in 3 separate steps, running sequentially; it
simplifies the code, but may be changed for efficiency/robustness in the next
iterations
* Periodic delivery collects per-actor keys in a
1-DB-transaction-for-each-delivery fashion, rather than grabbing them in the
big Esqueleto query (or keeping the signed output in the DB; this isn't done
currently to allow for smooth actor key renewal)
* No support yet in the API for delivery where the actor key has already been
fetched, rather than doing a DB transaction to grab it; such support would be
just an optimization, so it's low-priority, but will be added in later
iterations
