Registering a thread (associating it with a certain account name) would
fail if it was already registered. However, as we a) never unregister most
threads (bad) and b) single-threaded mode reuses threads, we failed when
syncing multiple accounts in single-threading mode.
This commit cleans up the functions to not make re-registering a thread
fatal (it could be legitimate, however it *should* not occur). Future
work needs to be done to unregister new threads at the appropriate places.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
8970a1500b