Add checkers in threads

[jbiason]

An attempt to solve #8. Instead of going with async, I tried to keep things in threads, as Flask (for example) does not use async and that would not work (how would one run an event loop on each thread?).

Also, I did some reorganization on the files, as (AFAIK) the default requirements files are called requirements.txt and requirements-dev.txt (again, AFAIK, but there is nothing forcing to use that name).

[luiscoms]

It is braking ateliedocodigo/eve-healthcheck, cause it shares database connection pool with health check

[2019-10-09 13:03:01,787] ERROR:healthcheck.healthcheck:73 - module 'eve' has no attribute 'SOURCES'
Traceback (most recent call last):
  File "/src/api/venv/lib/python3.5/site-packages/eve/utils.py", line 37, in __getattr__
    return app.config.get(name)
  File "/src/api/venv/lib/python3.5/site-packages/werkzeug/local.py", line 343, in __getattr__
    return getattr(self._get_current_object(), name)
  File "/src/api/venv/lib/python3.5/site-packages/werkzeug/local.py", line 302, in _get_current_object
    return self.__local()
  File "/src/api/venv/lib/python3.5/site-packages/flask/globals.py", line 51, in _find_app
    raise RuntimeError(_app_ctx_err_msg)
RuntimeError: Working outside of application context.

[luiscoms]

As documented here, a possible solution is use flask app context like:

def check_wrapper(app):
    def database_check():
        with app.app_context():
            # perform a count
            con = app.db.connect()
             if not:
                return False, "Database NOK"
            return True, "Database OK"
    return database_check

The secret here is the app context, that needs to be used as app.app_context(). The problem is that this library should work with flask and tornado, so it needs to be generic