Issue #28, #29 and #30

in class MySQL_Connection_Pool:
- shared converted from int to bool
- added 2 functions MySQL_Connection_lookup()

in class MyConnArray:
- match() and add() are not private
- class MySQL_Connection_Pool is a friend class
- added function MyConn_find()

in class MySQL_Connection:
- class MyConnArray is a friend class
- MCA is private
- removed custom operator new() and delete() because MySQL_Connection_Pool can be shared
- all members don't use the custom memory allocator
- added private function is_expired . See issue #30
- member expire removed, and replaced with inserted_into_pool