ShMem: Extract the mem access tasks & mem access latency encoding

* Define a separate encoding for memory access latencies that is derived
  form the task mapping. Also, add an according Decoder.
* Create an IMemAccessTask that wraps AF3 task only performing memory
  accesses.
* Add methods to obtain direct successors of tasks from TaskGraphs and
  rename the previoulsy existing transient methods.

Signed-off-by: Alexander Diewald <diewald@fortiss.org>