Upgrade Appveyor to VS2017

Summary:
Per some discussions, this will switch our Appveyor testing to use Visual Studio 2017.
Closes https://github.com/facebook/rocksdb/pull/3445

Differential Revision: D6874918

Pulled By: gfosco

fbshipit-source-id: c5a0032ca9f37f0d3baeae35c59d850d528c3176

appveyor.yml

@@ -1,9 +1,9 @@
 version: 1.0.{build}
-image: Visual Studio 2015
+image: Visual Studio 2017
 before_build:
 - md %APPVEYOR_BUILD_FOLDER%\build
 - cd %APPVEYOR_BUILD_FOLDER%\build
-- cmake -G "Visual Studio 14 Win64" -DOPTDBG=1 -DWITH_XPRESS=1 -DPORTABLE=1 ..
+- cmake -G "Visual Studio 15 Win64" -DOPTDBG=1 -DWITH_XPRESS=1 -DPORTABLE=1 ..
 - cd ..
 build:
   project: build\rocksdb.sln

db/external_sst_file_test.cc

@@ -388,9 +388,10 @@ class SstFileWriterCollector : public TablePropertiesCollector {
   const char* Name() const override { return name_.c_str(); }
 
   Status Finish(UserCollectedProperties* properties) override {
+    std::string count = std::to_string(count_);
     *properties = UserCollectedProperties{
         {prefix_ + "_SstFileWriterCollector", "YES"},
-        {prefix_ + "_Count", std::to_string(count_)},
+        {prefix_ + "_Count", count},
     };
     return Status::OK();
   }