continue the work for making multithreading easier

[Nicolas Sebrecht]

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