Initial Thread-Safe Implementation

Created by: ahayashi

Add thread-safety to XACC/QCOR (Experimental). We introduce _XACC_MUTEX macro in xacc.hpp to turn on/off the feature (default is OFF). When the macro is ON, only a single thread can execute the following routines by using eit
her std::mutex or std::recursive_mutex:

• User-facing API routines

  • createObjectiveFunction() (qcor::__internal__::get_objective())
  • createOptimizer()

• Compiler-related modules

  • Syntax Handler
    • QuantumKernel::operator()
    • QuantumKernel::~QuantumKernel()
  • QJIT
    • QJIT::QJIT()
    • QJIT::jit_compile()
    • QJIT::write_cache()

Also, regardless of the macro, the VQEObjective class is now xacc::Cloneable.

Add the following test files to test multi-thread execution. It is worth noting that, when the macro is OFF, these test programs are executed sequentially.

• examples/simple/bell_threaded.cpp
• examples/simple/simple-objective-function-async.cpp
• examples/simple/simple-objective-function-threaded.cpp
• examples/qpu_lambda/deuteron_threaded.cpp
• examples/qpu_lambda/deuteron_vqe_threaded.cpp
• examples/qpu_lambda/deuteron_vqe_obj_func_threaded.cpp
• examples/qpu_lambda/lambda_test_bell_threaded.cpp