Projector memory monitoring doesn't work correctly in memory limited containers

In 7.0, Projector added support for throttling mutations based on available system memory. This seems like difficult behavior to get right when the memory information you have is accurate and mostly up-to-date, but certainly becomes impossible if the memory measurements being taken are not giving the information you think they're giving.

This happens in memory limited containers. Memory limits are typically expressed by the memory.limit_in_bytes file in the appropriate cgroup and interpreting the information found there isn't straightforward and it's not something that sigar does at all currently.

Currently sigar returns system memory. If the container in which the Couchbase Server indexer node is running is not memory limited, the memory numbers returned by sigar should accurately reflect system total memory and (slightly out of date) accurate free, buffers and cached memory. If the container is memory limited, the numbers from sigar aren't meaningful for the container.