DcpProducer needs a consistent mechanism for access to the backfillMgr (shared_ptr)

Issue flagged from TSAN when adding new code to the DcpProducer.

In DcpProducer we have patterns like

if (backfillMgr) {

   backfillMgr->doThing();

}

Or even just unguarded calls

backfillMgr->doThing();

A new change for MB-57772 (which did the first pattern) triggered a TSAN warning, because DcpProducer::closeAllStreams will call reset on the backfillMgr shared_ptr - so clearly we could on one line read and believe the backfillMgr has a pointer, but on the second line crash when dereferencing a now nullptr.

T1                          |    T2

                            |

if (backfillMgr)            | 

                            |    backfillMgr.reset();

                            |

   backfillMgr->doThing();  |

   CRASHED                  | 

We need a safer way of managing access to the backfillMgr (e.g. folly::sync)