Marked statics as thread-safe since update only at library load

The singletons VertexRecoManager and VertexFitterManager are only
updated while the library in which they are embedded is loaded.
Therefore access to these singletons is thread-safe. By marking them
with our C++11 attribute, the static analyzer will no longer complain
about them.
Merge pull request #4428 from Dr15Jones/markStaticThreadSafeRecoVertexConfigurableVertexReco

Marked statics as thread-safe since update only at library load