Avoid copying FB303's get(Regex|Selected)Counters result map in BaseService.h

Summary:
## Context

I was glancing at the [Container Copies](https://www.internalfb.com/intern/unidash/dashboard/bad_patterns/container_copies/?dimensional_context_300674582190283=%7B%22macros%22%3A[]%2C%22operators%22%3A[]%2C%22movingAggregation%22%3A%22DEFAULT%22%2C%22granularity%22%3A%22DEFAULT%22%2C%22limit%22%3A5%7D) Bad Patterns dashboard and this one stood out to me. This is a locally defined `std::map` that we don't seem to use beyond the `result` call where I've added `std::move`.

Similar optimizations were performed in D55817430, where the underlying callback was made to take arguments by value. This must have been missed.

Seems like a small ([~14kW](https://fburl.com/scuba/strobelight_services/whv43t10)), but easy win.

Reviewed By: yfeldblum

Differential Revision: D68642453

fbshipit-source-id: 3d7687ae0d2c9a57304ea86c7b03538ee380f957

Author: Michael Poggy

fb303/BaseService.h

@@ -233,7 +233,7 @@ class BaseService : virtual public cpp2::BaseServiceSvIf {
           }
           addCountersAvailableToResponse(reqCtx, numAvailable);
         }
-        callback_->result(res);
+        callback_->result(std::move(res));
       } catch (...) {
         callback_->exception(std::current_exception());
       }
@@ -273,7 +273,7 @@ class BaseService : virtual public cpp2::BaseServiceSvIf {
           }
           addCountersAvailableToResponse(reqCtx, numAvailable);
         }
-        callback_->result(res);
+        callback_->result(std::move(res));
       } catch (...) {
         callback_->exception(std::current_exception());
       }
@@ -313,7 +313,7 @@ class BaseService : virtual public cpp2::BaseServiceSvIf {
           }
           addCountersAvailableToResponse(reqCtx, numAvailable);
         }
-        callback_->result(res);
+        callback_->result(std::move(res));
       } catch (...) {
         callback_->exception(std::current_exception());
       }