db: Don't blindly iterate the NULL context
We need to take the current threads default context into account as otherwise async operations started in that context might end up in the "main main" context due to libcmatrix blindly iterating that context when waiting for async operations to finish resulting in
libsoup:ERROR:../libsoup/soup-session.c:1458:message_completed: assertion failed: (item->context == soup_thread_default_context ()) Bail out! libsoup:ERROR:../libsoup/soup-session.c:1458:message_completed: assertion failed: (item->context == soup_thread_default_context ())
This is the same as https://source.puri.sm/Librem5/libcmatrix/-/merge_requests/64 but for chatty's history db.
Relates to: https://source.puri.sm/Librem5/libcmatrix/-/issues/24#note_271937
Edited by Guido Günther