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.cpp

@@ -245,6 +245,14 @@ bool CCoinsViewCache::HaveInputs(const CTransaction& tx) const
     return true;
 }
 
+void CCoinsViewCache::ReallocateCache()
+{
+    // Cache should be empty when we're calling this.
+    assert(cacheCoins.size() == 0);
+    cacheCoins.~CCoinsMap();
+    ::new (&cacheCoins) CCoinsMap();
+}
+
 static const size_t MIN_TRANSACTION_OUTPUT_WEIGHT = WITNESS_SCALE_FACTOR * ::GetSerializeSize(CTxOut(), PROTOCOL_VERSION);
 static const size_t MAX_OUTPUTS_PER_BLOCK = MAX_BLOCK_WEIGHT / MIN_TRANSACTION_OUTPUT_WEIGHT;