Merge #13577: logging: avoid nStart may be used uninitialized in AppInitMain warning

2dcd7b4ec logging: avoid nStart may be used uninitialized in AppInitMain warning (mruddy)

Pull request description:

  Was getting the following compiler warning:

  ```
  init.cpp: In function ‘bool AppInitMain()’:
  init.cpp:1616:60: warning: ‘nStart’ may be used uninitialized in this function [-Wmaybe-uninitialized]
           LogPrintf(" block index %15dms\n", GetTimeMillis() - nStart);
  ```

  It's ok without this PR, but this PR renames `nStart` to `load_block_index_start_time`, makes it `const`, and also reduces the scope of the variable.

  The logging line is moved such that the the time spent will be logged even if a shutdown is requested while the index is being loaded.

  Having the log message output even when a shutdown is requested may be how this was intended to work before anyways. That could explain the leading space, as such a log message now looks like:
  ```
  2018-06-30T11:34:05Z [0%]...[16%]...[33%]...[50%]... block index           25750ms
  2018-06-30T11:34:17Z Shutdown requested. Exiting.
  ```

Tree-SHA512: 967048afbc31f2ce8f80ae7d33fee0bdcbe94550cf2b5b662087e2a7cff14a8bf43d909b30f930660c184ec6c3c7e1302a84e3e54fc1723f7412827f4bf2c518

src/init.cpp

@@ -1295,8 +1295,6 @@ bool AppInitMain()
             return InitError(_("Unable to start HTTP server. See debug log for details."));
     }
 
-    int64_t nStart;
-
     // ********************************************************* Step 5: verify wallet database integrity
     if (!g_wallet_init_interface.Verify()) return false;
 
@@ -1448,8 +1446,8 @@ bool AppInitMain()
 
         LOCK(cs_main);
 
-        nStart = GetTimeMillis();
         do {
+            const int64_t load_block_index_start_time = GetTimeMillis();
             try {
                 UnloadBlockIndex();
                 pcoinsTip.reset();
@@ -1572,6 +1570,7 @@ bool AppInitMain()
             }
 
             fLoaded = true;
+            LogPrintf(" block index %15dms\n", GetTimeMillis() - load_block_index_start_time);
         } while(false);
 
         if (!fLoaded && !ShutdownRequested()) {
@@ -1601,9 +1600,6 @@ bool AppInitMain()
         LogPrintf("Shutdown requested. Exiting.\n");
         return false;
     }
-    if (fLoaded) {
-        LogPrintf(" block index %15dms\n", GetTimeMillis() - nStart);
-    }
 
     fs::path est_path = GetDataDir() / FEE_ESTIMATES_FILENAME;
     CAutoFile est_filein(fsbridge::fopen(est_path, "rb"), SER_DISK, CLIENT_VERSION);