[FTS] throttle http requests if using too much memory

See also MB-24823, where during system tests, the linux OOM killer comes along and kills cbft.

So, thoughts... we're regularly monitoring our memory usage already in cbft... https://github.com/couchbase/cbft/blob/master/ns_server.go#L337

Perhaps we can use that number to hook into your recent http listener / connection limiter changes?

One wrinkle is finding the right stat to use... https://golang.org/pkg/runtime/#MemStats

Another wrinkle is the memory usage sampling is pretty low frequency... we sample the go runtime memory usage only every minute (because performing the API call to get memory usage locks the go runtime)... https://github.com/couchbase/cbft/blob/master/ns_server.go#L896
...so a dynamic http throttling system might not have an ability to rapidly change its mind about its decisions.