ns_port_server does not release memory after the message queue backlog

As seen in a recent case:

1. Due to the high amount of messages the message queue of ns_port_server got backlogged and grew.
2. The heap grew to the quite a big size due to all the {data, Bin}

   tuples in the queue.

3. The global heap grew even more because all these tuples reference binaries.
4. Then the message stream was reduced to a trickle.
5. At some point while the queue backlog was clearing, the garbage collection happened, which freed approximately half of the heap, but the heap size was not reduced, because less than 75% of data was freed.
6. Now we have quite a big free chunk of the heap and quite slow heap allocation rate, so it could take quite a lot of time for the next garbage collection to happen.
7. Therefore we have tons of binaries stuck in the global heap.

the remedy: force garbage collection of ns_port_server periodically.