Refactor Invoker class with using queue

[rszamszur]

• I have searched the issues of this repo and believe that this is not a duplicate.

Feature Request

There is no need for a fixed amount of Invoker commands to execute (on_start, on_finish). A better solution would be a queue which would store and execute commands.

Implementation wise use collections.deque because of the constant time O(1) for poping and appending elements at both ends of the list (https://wiki.python.org/moin/TimeComplexity).

Related to: #51