preflightchecks.go

To better provide insight for customers as to whether the version of
DMD's tools can be used with a given version of a database, as well as
whether their SQL queries may work as-is, we can use the
`compatible_since` metadata.

This falls back to the existing compatibility heuristic - of checking if
there is an exact match between the CLI version in the database, and the
version of the tool in use - which is fairly naive.

In the case that `compatible_since` is present, we should present it to
the user, even if we can't validate whether `IsCompatible`.

As part of #532.