[3.0.1 Backport] One shot sub changes request does not decrement NumPullReplActiveOneShot stat when completed

The NumPullReplActiveOneShot stat is not decremented when a one shot sub changes request has finished pulling and sending the sub changes. It will decrement when closing the Blip client provided it has NOT been reused.

Reusing the Blip client causes the decrement code not to run as it is only ran when closing the client. This code is in function (bsc *BlipSyncContext) Close().
Running a one-shot before reusing the blip client for a continuous changes feed will cause the continuous to be replaced meaning that even when the Blip client is closed, the one shot active stats don't get decremented accordingly.

A potential solution to this is to decrement the one shot stat for Blip SubChangesOneShotActive (which is used for the db stat NumPullReplActiveOneShot) when the one shot has sent the results instead of doing it when the Blip client is closed.