Currently db will open when GetDBHandle method is invoked first time,
so GetDBHandle method needs to use mutex.
This patch makes db open and close in its lifecycle methods Start and
Stop, invokes Start only when peer starts and invokes Stop when peer
stops.
After that, methods in db do not need to support concurrency.

Change-Id: Id8612f2a846c5d626bd42c5cd4ae482076f6a975
Signed-off-by: grapebaba <281165273@qq.com>