client: Don't create unused task and other small improvements

Finalizing a task that never invoked g_task_return* leads to a critical (if a callback is set) or a debug message (when no callback is set). The later is true for matrix_start_sync() when the "*_loading" check is TRUE.

As the task is not used for anything avoid this printout which looks scary as it can happen periodically.

Note that the branches

if (self->db_migrated && !self->{direct_,}room_list_loaded)

don't use task and hence can still trigger the warning.

The other commits move to g_timeout_add_seconds_once and fix some indent.