Basic CREATE USER implementation

[Hydrocharged]

This adds the minimum for the CREATE USER statement to work, in that we have an in-memory representation of the grant tables, which may be accessed either directly (by the mysql database) or through the user statements (of which we only have CREATE USER for now).

[zachmu]

This seems like a wrong turn, but I could be convinced otherwise.

There's no need to keep all physical grant / user rows in memory all the time. What you need is

1. A Database interface extension for CheckPrivileges (this is what your analyzer function will call into)
2. A DatabaseProvider interface extension for GetUsers / GetGrants etc., not sure exactly what's needed there

The table representation is only really relevant when querying the user / grant tables directly. And I don't know if we will ever care if that's fast, I'm totally fine constructing them from the primitives in 2) at runtime as needed. The thing that needs to be fast is auth during analysis, and for that you should provide custom hooks for integrators that they can make as fast as necessary.

This is the same pattern we have followed for all the other mysql system tables: we build them at query time from the information provided by integrators through the interfaces they implement. I don't see a good reason to deviate from that pattern for this.

Let me know if I have the wrong idea here.

[zachmu]

Seems generally fine, hard to give more concrete feedback until I see how it ties into an integrator