JSON-RPC: retrieve events via long polling

This way is more compatible to JSON-RPC libraries
that do not support receiving notifications from the server
and allows describing event types in the OpenRPC specification.

Event thread converting events to notifications in the FFI
is removed, so it is now possible to construct a dc_jsonrpc_instance_t
while still retrieving events via dc_event_emitter_t.

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 ## Unreleased
 
+### Changes
+- JSON-RPC: Use long polling instead of server-sent notifications to retrieve events.
+  This better corresponds to JSON-RPC 2.0 server-client distinction
+  and is expected to simplify writing new bindings
+  because dispatching events can be done on higher level.
+
 ### Fixes
 - JSON-RPC: do not print to stdout on failure to find an account.