refactor cache decorator

[wuwen5]

Reopen from #838 and related #2921

[coveralls]

coverage: 87.144% (+0.03%) from 87.116%
when pulling 0379d03 on wuwen5:refactor-cache-decorate
into 4c9d63d on mybatis:master.

[harawata]

I expressed my concern about allowing direct access to the wrapped instances.
My personal opinion is: if you want to see the cache statistics, you should use some third party cache implementation instead of MyBatis' internal cache.

[wuwen5]

• The access level of the getDelegate() has been modified to protected.
• Removed the getHits() from LoggingCache

[harawata]

@wuwen5 ,

I'm sorry for the belated response.
I have taken a look at this several times actually, but I still prefer keeping these cache implementations loosely coupled.

@hazendaz ,
If you like this, feel free to merge.
I won't object.

[hazendaz]

Will circle back on this in some time. I think the general idea was that users wanted it and I believe I suggested it was ok to create new PR bringing others up to date. I do not have any strong preference either way.