clear out snapshots after loading them

They're only needed as long as the server is down. This fixes a bug where
snapshots are saved, the server comes up, and then a container is destroyed,
and then the server is restarted. Previously the old snapshot would stick
around, and the server would fail to recover it.
add logging for snapshot saving/loading
reap containers that are idle beyond grace time

The server can be configured with a global default grace time via a flag,
otherwise the value given in the Create message is respected.

Note: the grace time given to a container on creation, whether it's via the
message or the server default, is persisted in the snapshot. This means that
if the default value is changed, previous containers are *not* switched to the
new default.

[finishes #61232348]