func (sw *sessionWrapper)DB(name string) *mgo.Database {
	return sw.s.DB(name)
}
only allowed native session operation
package mgop
import (
	"gopkg.in/mgo.v2"
	"sync/atomic"
)
type sessionWrapper struct {
	s        *mgo.Session
	belongTo SessionPool
	ref      int32
	maxRef   int32
	host     string  
}
func (sw *sessionWrapper) Release() {
	sw.ReleaseWithFraction(-1)
}
func (sw *sessionWrapper) ReleaseWithFraction(i int32) {
	atomic.AddInt32(&sw.ref, i)
}
func (sw *sessionWrapper) atomicAcquire() bool {
	atomic.AddInt32(&sw.ref, 1)
	return true
}