Initial algorithm for nominating valid 'parallel' tx sets.

This is a relatively simple algorithm, but it should be serviceable for traffic with relatively low amount of transitive IO conflicts.

Author: dmkozh

Builds/VisualStudio/stellar-core.vcxproj

@@ -522,6 +522,7 @@ exit /b 0
     <ClCompile Include="..\..\src\herder\HerderSCPDriver.cpp" />
     <ClCompile Include="..\..\src\herder\HerderUtils.cpp" />
     <ClCompile Include="..\..\src\herder\LedgerCloseData.cpp" />
+    <ClCompile Include="..\..\src\herder\ParallelTxSetBuilder.cpp" />
     <ClCompile Include="..\..\src\herder\PendingEnvelopes.cpp" />
     <ClCompile Include="..\..\src\herder\QuorumIntersectionCheckerImpl.cpp" />
     <ClCompile Include="..\..\src\herder\QuorumTracker.cpp" />
@@ -986,6 +987,7 @@ exit /b 0
     <ClInclude Include="..\..\src\herder\HerderSCPDriver.h" />
     <ClInclude Include="..\..\src\herder\HerderUtils.h" />
     <ClInclude Include="..\..\src\herder\LedgerCloseData.h" />
+    <ClInclude Include="..\..\src\herder\ParallelTxSetBuilder.h" />
     <ClInclude Include="..\..\src\herder\PendingEnvelopes.h" />
     <ClInclude Include="..\..\src\herder\QuorumIntersectionChecker.h" />
     <ClInclude Include="..\..\src\herder\QuorumIntersectionCheckerImpl.h" />

Builds/VisualStudio/stellar-core.vcxproj.filters

@@ -1397,6 +1397,9 @@
     <ClCompile Include="..\..\src\transactions\LumenEventReconciler.cpp">
       <Filter>transactions</Filter>
     </ClCompile>
+    <ClCompile Include="..\..\src\herder\ParallelTxSetBuilder.cpp">
+      <Filter>herder</Filter>
+    </ClCompile>
   </ItemGroup>
   <ItemGroup>
     <ClInclude Include="..\..\lib\util\cpptoml.h">
@@ -2474,6 +2477,9 @@
     <ClInclude Include="..\..\src\transactions\LumenEventReconciler.h">
       <Filter>transactions</Filter>
     </ClInclude>
+    <ClInclude Include="..\..\src\herder\ParallelTxSetBuilder.h">
+      <Filter>herder</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <None Include="..\..\AUTHORS" />