Refactor conversion node

[lreiher]

• refactor conversion node for better extensibility to more message types
• add parameter for subscriber/publisher queue size
• all message types were successfully tested in conversion node

[lreiher]

Somehow this also solves all remaining memory leaks after #21.

==381575== HEAP SUMMARY:
==381575==     in use at exit: 505,389 bytes in 139 blocks
==381575==   total heap usage: 9,105 allocs, 8,966 frees, 14,526,878 bytes allocated
==381575== 
==381575== LEAK SUMMARY:
==381575==    definitely lost: 0 bytes in 0 blocks
==381575==    indirectly lost: 0 bytes in 0 blocks
==381575==      possibly lost: 0 bytes in 0 blocks
==381575==    still reachable: 505,389 bytes in 139 blocks
==381575==         suppressed: 0 bytes in 0 blocks
==381575== Reachable blocks (those to which a pointer was found) are not shown.
==381575== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==381575== 
==381575== For lists of detected and suppressed errors, rerun with: -s
==381575== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)