Add accounting implementation for type "stop"

* rlm_mongodb.c (mongodb_accounting_{pre}stop):
  - Implementation of accounting type "stop".
  - It could re-generate the record if no start before the stop and
    only if the Acct-Session-Time is specified, otherwise the operation
    should fail.
  - The Acct-Session-{Start,Stop}-Time is in milliseconds,
    the Acct-Session-Time is in seconds.
* rlm_mongodb.c (mongodb_accounting_update):
  - Add flags as the additional parameter which the caller could pass the
    update flags individualy.
* rlm_mongodb.c (mongodb_accounting_start):
  - Adjust to follow the mongodb_accounting_update() parameters changes.

Signed-off-by: Neutron Soutmun <neo.neutron@gmail.com>
To prevent memleaks should free mongo-wire-packet on cursor creation failed

Signed-off-by: Neutron Soutmun <neo.neutron@gmail.com>