stmt should implement driver.StmtExecContext and driver.StmtQueryContext

[cespare]

This is a follow-on to #535.

For the driver.Stmt interface, the Exec and Query methods are deprecated. Drivers should instead (or additionally) implement ExecContext and QueryContext.

I couldn't find an existing issue or pull request regarding this. Apologies if I missed it.

/cc @mjibson

[maddyblue]

See #729. We will start by removing support for Go < 1.8, then we should be able to fully remove the deprecated methods and types.

[cespare]

Just to be clear, the problem here is that stmt doesn't implement the new methods, not that it does implement the deprecated methods.

[cespare]

Just to note the reason I ran into this: I have some code which needs to make use of a driver.Stmt directly and it calls stmt.Exec and stmt.Query. That triggers the deprecation checks in staticcheck (https://staticcheck.io/docs/staticcheck). I can't change my code to call stmt.ExecContext and stmt.QueryContext because pq.stmt doesn't implement those yet.

(It's not a big deal because I can filter out these warnings if I need to.)

[kylejbrock]

@cespare & @mjibson Not sure if this helps, but here's this: #768