Memory leak is still present

We had memory issues like #910 (closed), #961 (closed), #971 (closed).

Even if the memory leak seems fixed, Duniter still has a high memory usage (more than 1GB in RAM).

However when we make a heapdump, we can see only 32MB or RAM usage. This could be explained by Node.js long term memory usage, where Node does not clean its unused objects with Garbage Collector very often, and expands its memory usage as long as GC is not required.

We could set an upper limit for long term memory to 300 MB for example, as it seems to be enough for Duniter to work correctly.

I've tested such value, and Duniter uses ~100MB memory permanently.