api, examples, tests, benchmarks: react_context removed from public api

- Now react_context_t is not exposed to used and is kept as thread local
  variable in react.cpp

- Several react_activate functions can be called consequently and each
  call will increment refcount. Each react_deactivate will decrement
  refcount. When refcount will be zero, thread_local react_context will
  be removed.
v2.0.0