Ensure foreign memory is freed or gc'ed in qvm-app-ng

[appleby]

As reported in #198, large foreign allocations may not trigger a full gc and therefore result in an out-of-memory condition when the next request is processed.

A temporary fix was provided for qvm-app in #199 and #204. There is also some discussion in #200 about alternative solutions.

This issue tracks addressing #198 in qvm-app-ng as well.