core: various changes and fixes to work with new storage and D-Bus bridge: - fixes coroutines handling in various places - fixes types which are not serialised by Tx DBus - XEP-0384: call storage methods in main thread in XEP: Python OMEMO's Promise use thread which prevent the use of AsyncIO loop. To work around that, callLater is used to launch storage method in main thread. This is a temporary workaround, as Python OMEMO should get rid of Promise implementation and threads soon.