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
Backport of Core PR13577