continue the work for making multithreading easier

[Łukasz Żarnowiecki]

Hi Nicolas,

> Łukasz, I wonder my patches are making your job harder if you have WIP. Let me
> know so I stop while you're still working on Python 3 support.
> 
>  [PATCH 1/4] threading: simplify names
>  [PATCH 2/4] threading: rename threadslist to accountThreads
>  [PATCH 3/4] get rid of offlineimap/syncmaster.py
>  [PATCH 4/4] threading: get rid of the syncaccount function

I think those patches does not introduce any incompatible changes so
feel free to apply them atop "next" branch, but honestly I did not test
them only I took a look at the code.

If you could, please next time generate git request-pull(1) so I can
easily apply changes on my current work and test them immediately.  You
can even use "-p" option to attach diff.

Thanks
Łukasz