MINOR: spoe: Add support for pipelining/async capabilities in the SPOA example

MINOR: spoe: Add support for pipelining/async capabilities in the SPOA example

Now, we can use the option '-c' to enable the support of a capability. By
default, all capabilities are disabled. For example:

  $> ./spoa -c async -c pipelining

In addition, it is also possible to set the maximum frame size supported by your
agent (-m) and to add a delay in frames processing (-t).