fix: update context handling in *Context db methods to stop context pollution

[jsteenb2]

this is a fix for QueryContext, ExecContext context pollution. It also addresses PrepareStmt as that is a different lifecycle. Gorm makes heavy use of PrepareStmt. With this we have the ability to issue individual contexts for a {Query|Exec|Prepare}Context and cancel them. AKin to the following:

func doThingWithin3Seconds(ctx context.Context, db *sql.DB) {
     ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
     // cancels regardless of how the db method below results
     // should not affect future queries to the db if its successful though
     defer cancel() 
    
    // ... trim

    rows, err := db.QueryContext(....) // or ExecContext
    // ... trim
}

this fixes it in our old legacy gorm tests and our newer pgx|slqx tests.

issue: #43

[jsteenb2]

inlining this interface to avoid calling into context.Context, which would break backwards compatability with versions of go that predate the context pkg

[l3pp4rd]

thanks, sorry I saw your email, but forgot to have a look.