Bound warm workers' host memory and account local RAM

A worker over FLOW_WORKER_MAX_RSS_MB (--worker-max-rss) when its node
returns is retired instead of kept warm, in the shared pool and in every
declared one; 0, the default, keeps them all. The engine now accounts its
own memory (FLOW_RAM / --ram, else the box or the cgroup limit less 1 GB),
so a node's `ram` declaration holds locally. A declared environment the
shared pool already has is that pool rather than a duplicate set of warm
workers.

docs/code/cli.md

@@ -79,6 +79,8 @@ Another instance's Fluksio on the port is named, and the move happens as usual.
 | `--max-cascades N` | 4 | cascades in flight at once (`FLOW_MAX_CASCADES`) |
 | `--max-workers N` | 4 | python worker processes (`FLOW_MAX_WORKERS`) |
 | `--gpus N` | counted | GPUs on this machine a node may be given (`FLOW_GPUS`) |
+| `--ram MB` | worked out | memory on this machine a node may be given (`FLOW_RAM`) |
+| `--worker-max-rss MB` | 0 (never) | retire a warm worker holding more when its node returns (`FLOW_WORKER_MAX_RSS_MB`) |
 
 Cards are counted from NVIDIA's device nodes (`/dev/nvidia0`, `/dev/nvidia1`, …),
 which asks no vendor tool and so keeps the one dependency from becoming two.