Write witness caches when writing the best block

For steady-state operation, this reduces the average time between wallet disk
writes from once per block to once per hour.

On -rescan, witness caches are only written out at the end along with the best
block, increasing speed while ensuring that on-disk state is kept consistent.

Witness caches are now never recreated during a -reindex, on the assumption that
the blocks themselves are not changing (the chain is just being reconstructed),
and so the witnesses will remain valid.

Part of #1749.

src/init.cpp

@@ -537,11 +537,6 @@ void ThreadImport(std::vector<boost::filesystem::path> vImportFiles)
     RenameThread("zcash-loadblk");
     // -reindex
     if (fReindex) {
-#ifdef ENABLE_WALLET
-        if (pwalletMain) {
-            pwalletMain->ClearNoteWitnessCache();
-        }
-#endif
         CImportingNow imp;
         int nFile = 0;
         while (true) {
@@ -1379,7 +1374,10 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
 
         CBlockIndex *pindexRescan = chainActive.Tip();
         if (GetBoolArg("-rescan", false))
+        {
+            pwalletMain->ClearNoteWitnessCache();
             pindexRescan = chainActive.Genesis();
+        }
         else
         {
             CWalletDB walletdb(strWalletFile);