Add CChainState::ResizeCoinsCaches

Also adds CCoinsViewCache::ReallocateCache() to attempt to free
memory that the cacheCoins's allocator may be hanging onto when
downsizing the cache.

Adds `CChainState::m_coins{tip,db}_cache_size_bytes` data members
so that we can reference cache size on a per-chainstate basis for
flushing.

src/coins.h

@@ -318,6 +318,13 @@ public:
     //! Check whether all prevouts of the transaction are present in the UTXO set represented by this view
     bool HaveInputs(const CTransaction& tx) const;
 
+    //! Force a reallocation of the cache map. This is required when downsizing
+    //! the cache because the map's allocator may be hanging onto a lot of
+    //! memory despite having called .clear().
+    //!
+    //! See: https://stackoverflow.com/questions/42114044/how-to-release-unordered-map-memory
+    void ReallocateCache();
+
 private:
     /**
      * @note this is marked const, but may actually append to `cacheCoins`, increasing