Fix clang warnings about copies vs references:

A recent version of clang notes a number of places in range
for loops where the code base was making unnecessary copies
or using const lvalue references to extend lifetimes.  This
fixes the places that clang identified.

src/ripple/app/misc/impl/ValidatorList.cpp

@@ -99,7 +99,7 @@ void
 ValidatorList::PublisherListStats::mergeDispositions(
     PublisherListStats const& src)
 {
-    for (auto const [disp, count] : src.dispositions)
+    for (auto const& [disp, count] : src.dispositions)
     {
         dispositions[disp] += count;
     }