Optimize the using of logger for python class Cache and ApproximateTimeSynchronizer

[gaoethan]

This is the follow-up to the future optimization of the logger using in #7 for the following comment from @tfoote , now it's using exclusive named logger instead of the root logger within the initial porting.

However we've been looking at this elsewhere and for useful logging we will likely need to pass in the logger objects so that they can be remapped and otherwise grouped appropriately. So adding the logger to the init signature wouldn't be unreasonable. We can ticket that as a follow up.