scripted-diff: Avoid temporary copies when looping over std::map

The ::value_type of the std::map/std::multimap/std::unordered_map containers is
std::pair<const Key, T>. Dropping the const results in an unnecessary copy,
for example in C++11 range-based loops.

For this I started with a more general scripted diff, then narrowed it down
based on the inspection showing that all actual map/multimap/unordered_map
variables used in loops start with m or have map in the name.

-BEGIN VERIFY SCRIPT-
sed -i -E 's/for \(([^<]*)std::pair<([^c])(.+) : m/for (\1std::pair<const \2\3 : m/' src/*.cpp src/**/*.cpp
sed -i -E 's/for \(([^<]*)std::pair<([^c])(.+) : (.*)map/for (\1std::pair<const \2\3 : \4map/' src/*.cpp src/**/*.cpp
-END VERIFY SCRIPT-

src/init.cpp

@@ -614,7 +614,7 @@ static void CleanupBlockRevFiles()
     // keeping a separate counter.  Once we hit a gap (or if 0 doesn't exist)
     // start removing block files.
     int nContigCounter = 0;
-    for (const std::pair<std::string, fs::path>& item : mapBlockFiles) {
+    for (const std::pair<const std::string, fs::path>& item : mapBlockFiles) {
         if (atoi(item.first) == nContigCounter) {
             nContigCounter++;
             continue;