[wip,dnr,dnm] server,cli: bar decommissioned nodes from re-joining the cluster

[irfansharif]

*: persist a prevent startup file on decomm

Does not work for decommissioning non-live nodes. Does not actually get
checked either. Not sure if we want to use a file as such, or a store
local key. We're also arbitrarily using the first store to do such a
thing (should we just write it to every store?).

Release note: None

---

*: consult gating file on start up

And introduce (broken) --force flag to `cockroach node decommission`).

Release note: None

[cockroach-teamcity]

This change is 

[irfansharif]

Abandoning this PR, @tbg is instead going to attempt the following approach:

We'll install a gossip listener near pkg/rpc that will listen in on changes to liveness records. When this listener learns of that a node is fully decommissioned, it will persist that information to a store local key/file. That file/information (also cached in-memory) will be checked in our rpc layer when heartbeating currently open connections, and when accepting new ones. This will effectively let us close out all connections to all fully decommissioned nodes. This file will also be checked during start up to populate our cache, and to maintain the running list of "nodes we shouldn't talk to anymore".

+cc @knz, this overlaps with areas I think you were planning on otherwise working on.