Allow per message compression for websocket

[scottinet]

Feature description

Per message compression is currently explicitly deactivated server-side due to catastrophic memory fragmentation problems encountered in Node.js when heavily using zlib (nodejs/node#8871).

A solution seems to have been found and merged a few days ago (nodejs/node#34048). We need to keep an eye on it: if this lands in Node.js 12, we could then allow per-message compression in websockets.

If we do though, this would need to be deactivated by default. A new flag in Kuzzle's configuration files would allow to activate that feature, with a proper documentation explaining from which version of node.js that option can be safely used.