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/validation.h

@@ -127,7 +127,6 @@ extern bool g_parallel_script_checks;
 extern bool fRequireStandard;
 extern bool fCheckBlockIndex;
 extern bool fCheckpointsEnabled;
-extern size_t nCoinCacheUsage;
 /** A fee rate smaller than this is considered zero fee (for relaying, mining and transaction creation) */
 extern CFeeRate minRelayTxFee;
 /** If the tip is older than this (in seconds), the node is considered to be in initial block download. */
@@ -519,7 +518,7 @@ public:
 
     //! Initialize the in-memory coins cache (to be done after the health of the on-disk database
     //! is verified).
-    void InitCoinsCache() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+    void InitCoinsCache(size_t cache_size_bytes) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     //! @returns whether or not the CoinsViews object has been fully initialized and we can
     //!          safely flush this object to disk.
@@ -568,6 +567,17 @@ public:
     //! Destructs all objects related to accessing the UTXO set.
     void ResetCoinsViews() { m_coins_views.reset(); }
 
+    //! The cache size of the on-disk coins view.
+    size_t m_coinsdb_cache_size_bytes{0};
+
+    //! The cache size of the in-memory coins view.
+    size_t m_coinstip_cache_size_bytes{0};
+
+    //! Resize the CoinsViews caches dynamically and flush state to disk.
+    //! @returns true unless an error occurred during the flush.
+    bool ResizeCoinsCaches(size_t coinstip_size, size_t coinsdb_size)
+        EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     /**
      * Update the on-disk chain state.
      * The caches and indexes are flushed depending on the mode we're called with