* Avoid releasing symbol manager. Let OS release resources
* Does not affect leak sanitizer
* Use static instead of thread local to avoid loading the library
multiple times
* Move activeBackend and activeHandle out of symbol manager since
AFSymbolManager is a singleton class now. It didn't make sense
to have those variables in there anyway because they do not do
anything with the symbols. Added multi-threaded tests to make
sure expected behavior when switching backends
multiple times