SWG: Collect addiction information at high memory usage

Problem

There are time when SyncGateway uses a high amount of memory and is killed by OOM-Killer. Unfortunately in these cases it can be hard to work out what is using so much memory. Normally we can get to the bottom of this cases once we get a pprof, that's impossible after the facts and our users are forced to reproduce the issue

Suggestion

Here are a few suggestions to over come this issue:

• Collect pprof at regular interval and keep the last 5
• Collect pprof when SGW is using 90% of system memory