More bestblock records in wallets

Write bestblock records in wallets:
* Every 20160 blocks synced, no matter what (before: none during IBD)
* Every 144 blocks after IBD (before: for every block, slow)
* When creating a new wallet
* At shutdown

This should result in far fewer spurious rescans.

src/init.cpp

@@ -100,6 +100,7 @@ void Shutdown()
     StopNode();
     {
         LOCK(cs_main);
+        pwalletMain->SetBestChain(CBlockLocator(pindexBest));
         if (pblocktree)
             pblocktree->Flush();
         if (pcoinsTip)
@@ -998,6 +999,8 @@ bool AppInit2(boost::thread_group& threadGroup)
             if (!pwalletMain->SetAddressBookName(pwalletMain->vchDefaultKey.GetID(), ""))
                 strErrors << _("Cannot write default address") << "\n";
         }
+
+        pwalletMain->SetBestChain(CBlockLocator(pindexBest));
     }
 
     printf("%s", strErrors.str().c_str());

src/main.cpp

@@ -1870,7 +1870,7 @@ bool SetBestChain(CValidationState &state, CBlockIndex* pindexNew)
     }
 
     // Update best block in wallet (so we can detect restored wallets)
-    if (!fIsInitialDownload)
+    if ((pindexNew->nHeight % 20160) == 0 || (!fIsInitialDownload && (pindexNew->nHeight % 144) == 0))
     {
         const CBlockLocator locator(pindexNew);
         ::SetBestChain(locator);