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/test/util/setup_common.cpp

@@ -139,7 +139,7 @@ TestingSetup::TestingSetup(const std::string& chainName, const std::vector<const
     ::ChainstateActive().InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
     assert(!::ChainstateActive().CanFlushToDisk());
-    ::ChainstateActive().InitCoinsCache();
+    ::ChainstateActive().InitCoinsCache(1 << 23);
     assert(::ChainstateActive().CanFlushToDisk());
     if (!LoadGenesisBlock(chainparams)) {
         throw std::runtime_error("LoadGenesisBlock failed.");