smp pes sending messages still block due to other send activity
Despite #642 being merged I still see pes that are sending messages blocked waiting on some shared resource in smp builds.
Attached is a communication torture test based on PME in NAMD that illustrates the problem quite clearly.
It is desirable for the sending pe to do anything that the communication thread does not absolutely need to do itself, but it shouldn't get stuck doing nothing.
Note that in the attached timeline screenshot the pink entries all send the same number and size of messages.