This commit introduces a few improvements to the `mixedversion`
package, the recently introduced framework for mixed-version (and
mixed-binary) roachtests.

Specifically, the following improvements are made:

* Removal of `DBNode()` function: having the planner pick a database
that the individual steps will connect to is insufficient in many
cases and could be misleading. The idea was that the user would be
able to see, from the test plan itself, what node a certain step would
be interacting with. However, the reality is that steps often need to
run statements on multiple different nodes, or perhaps they need to
pick one node specifically (e.g., the statement needs to run on a node
in the old version). For that reason, the `DBNode()` function was
dropped. Instead, steps have access to a random number generator that
they can use to pick an arbitrary node themselves. The random number
generators are unique to each user function, meaning each test run
will see the same numbers being generated even if other steps are
scheduled concurrently. The numbers observed by a user function will
also be the same if the seed passed to `mixedversion.Test` is the
same.

* Definition of a "test context" that is available to mixed-version
tests. For now, the test context includes things like which version we
are upgrading (or downgrading) to and from and which nodes are running
which version. This allows tests to take actions based on, for
example, the number of nodes upgraded. It also allows them to run
certain operations on nodes that are known to be in a specific
version.

* Introduction of a `helper` struct that is passed to user-functions.
For now, the helper includes functions to connect to a specific node
and get the current test context. The struct will help us provide
common functionality to tests so that they don't have to duplicate
code.

* Log cached binary and cluster versions before executing a step. This
makes it easier to understand the state of the cluster when looking at
the logs of one specific step.

* Internal improvement to the test runner: instead of assuming the
first step of a mixed-version test plan will start the the cockroach
nodes, we now check that that is the case, providing a clear error
message if/when that assumption doesn't hold anymore (instead of a
cryptic connection failure error).

Epic: CRDB-19321

Release note: None