27922: storage: add closed timestamp subsystem r=nvanbenschoten a=tschottdorf

NB: This is a reopen of #27595 which needs a fix to the CI stress tester
to merge before it can pass CI (and I don't want to wait for that). No
need to review.

----

This commit introduces the basic closed timestamp subsystem. It does
not hook it up into the CockroachDB node yet.

Great care has been taken to make each individual component self-
contained and cleanly interfaced to the outside world. The package
layout relative to `closedts` is the following:

.: slim root package containing only the most basic CT interfaces
./ctpb: protobuf definitions and service
./config: fat package pulling in everything needed to actually run
./storage: storage for per-node closed timestamp state
./minprop: minimum proposal timestamp tracker
./provider: the Provider, centralizing all closed timestamp information.

Most of these packages depend on `closedts` (more precisely, on the
interfaces they need to use), but in turn don't depend on each other.

The following diagram together with the `ctconfig` and `ct` packages
should be instructive in getting a bird's eye view of how the subsystem
fits into CockroachDB: All of the components are grouped in a Container
(intended as a pass-around per-instance Singleton) and is passed down to
the Stores via the StoreConfig. Replicas proposing commands talk to the
Tracker; replicas trying to serve follower reads talk to the Provider,
which receives closed timestamp updates for the local node and its
peers.

```
                            Node 1 | Node 2
                                   |
+---------+  Close  +-----------+  |  +-----------+
| Tracker |<--------|           |  |  |           |
+-----+---+         | +-------+ |  |  | +-------+ |  CanServe
      ^             | |Storage| |  |  | |Storage| |<---------+
      |             | --------+ |  |  | +-------+ |          |
      |Track        |           |  |  |           |     +----+----+
      |             | Provider  |  |  | Provider  |     | Follower|
      |             +-----------+  |  +-----------+     | Replica |
      |                 ^                  ^            +----+----+
      |                 |Subscribe         |Notify           |
      |                 |                  |                 |
+---------+             |      Request     |                 |
|Proposing| Refresh +---+----+ <----- +----+-----+  Request  |
| Replica |<--------| Server |        |  Clients |<----------+
+---------+         +--------+ -----> +----------+  EnsureClient
                                 CT
```

Release note: None

Co-authored-by: Tobias Schottdorf <tobias.schottdorf@gmail.com>