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/validation.cpp

@@ -3840,7 +3840,7 @@ bool CChainState::LoadBlockIndex(const Consensus::Params& consensus_params, CBlo
     // Calculate nChainWork
     std::vector<std::pair<int, CBlockIndex*> > vSortedByHeight;
     vSortedByHeight.reserve(mapBlockIndex.size());
-    for (const std::pair<uint256, CBlockIndex*>& item : mapBlockIndex)
+    for (const std::pair<const uint256, CBlockIndex*>& item : mapBlockIndex)
     {
         CBlockIndex* pindex = item.second;
         vSortedByHeight.push_back(std::make_pair(pindex->nHeight, pindex));
@@ -3907,7 +3907,7 @@ bool static LoadBlockIndexDB(const CChainParams& chainparams)
     // Check presence of blk files
     LogPrintf("Checking all blk files are present...\n");
     std::set<int> setBlkDataFiles;
-    for (const std::pair<uint256, CBlockIndex*>& item : mapBlockIndex)
+    for (const std::pair<const uint256, CBlockIndex*>& item : mapBlockIndex)
     {
         CBlockIndex* pindex = item.second;
         if (pindex->nStatus & BLOCK_HAVE_DATA) {