blockstorage: Make m_block_index own CBlockIndex's

Instead of having CBlockIndex's live on the heap, which requires manual
memory management, have them be owned by m_block_index. This means that
they will live and die with BlockManager.

A change to BlockManager::LookupBlockIndex:
- Previously, it was a const member function returning a non-const CBlockIndex*
- Now, there's are const and non-const versions of
  BlockManager::LookupBlockIndex returning a CBlockIndex with the same
  const-ness as the member function:
    (e.g. const CBlockIndex* LookupBlockIndex(...) const)

See next commit for some weirdness that this eliminates.

The range based for-loops are modernize (using auto + destructuring) in
a future commit.

src/wallet/test/wallet_tests.cpp

@@ -367,10 +367,10 @@ static int64_t AddTx(ChainstateManager& chainman, CWallet& wallet, uint32_t lock
     CBlockIndex* block = nullptr;
     if (blockTime > 0) {
         LOCK(cs_main);
-        auto inserted = chainman.BlockIndex().emplace(GetRandHash(), new CBlockIndex);
+        auto inserted = chainman.BlockIndex().emplace(std::piecewise_construct, std::make_tuple(GetRandHash()), std::make_tuple());
         assert(inserted.second);
         const uint256& hash = inserted.first->first;
-        block = inserted.first->second;
+        block = &inserted.first->second;
         block->nTime = blockTime;
         block->phashBlock = &hash;
         state = TxStateConfirmed{hash, block->nHeight, /*position_in_block=*/0};