Added a DBCacheMap to replace std::map
This map can also mark DB intention so it's more professional to be a
DB cache structure
1. Fixed bugs of DBCacheMap
2. Make DBCache singleton
Changed test call to suit singleton DBCache
TODO: Correct sql statements to make it compatible with MySQL
Corrected previous error
Removed previous skeleton as they are too ugly
Adjusted main source structure.
TODO:solve the issue of different Socket used in Session