Fix mongodb node bug

  release connection when it is unused. service instances monitor
  calls finish_start? periodically, if connection not released then
  mongodb instance connection number shall increase dramatically,
  and finally exceeds 'maxConn', afterwards, clients will never
  connect to mongodb instance.

Change-Id: Idc59b3361aae9f146517c1d1dadb1e11df112c07
Merge "Fix mongodb node bug"