Author: Sijie Guo <email@example.com>
Date: Thu Apr 18 17:54:55 2013 -0700
make logger as a static variable
not all logger in bookkeeper & hedwig are static. some class like PendnigReadOp and LedgerEntry would have lots of objects, it might be bad. so this task is to make logger as a static variable if it didn't.
with benefit, cleaning up the imports when touching that file.