And set the `time.Time` option to use the go-msgpack-1.1.5-compatible
encoding in all the places, since that is the (now previous) version
in `go.mod`.

v2 2.1.1 was specifically designe to honor backwards compatibility
with 1.1.5 and 0.5.5, and to clean up the code base to be more
maintainable. There may performance lost with the 1.1.5 to 2.1.1
migration since the fastpath code was removed, but the increased safety
is probably worth it. See
[the release notes for go-msgkack 2.1.0](https://github.com/hashicorp/go-msgpack/releases/tag/v2.1.0)
for more details.

I tested this by running this code, and booting up a cluster with
a node also running the prior version of Nomad (before the upgrade).
Before I made the changes to set the right `time.Time` option, the
previous-version node would throw a bunch of time-decoding errors.
After fixing the option, the node came up smoothly, even after
changing leadership between them.

This relies on
- [ ] hashicorp/serf#705
- [ ] hashicorp/raft-boltdb#38
- [ ] hashicorp/raft#577
- [ ] hashicorp/memberlist#292

and maybe
- [ ] hashicorp/net-rpc-msgpackrpc#12