db: fix finalization problems

A `this%` expression used in a finalization callback implicitly
referred to `this`, since it's a dynamic reference to the object's
class. As a result, the finalizer for `this` refers to `this`, so
`this` never becomes collectable. The problem is fixed by
lifting the `this%` out of the `lambda`.

Less significantly, the finalizer thread in "prepared.rkt" captured
various parameters on creation, including the current namespace. If a
prepared statement is bound to a module-level variable, then the
finalizer thread refers through the namespace to the prepared
statement, so the prepared statement can never be finalized. Setting
the current namespace to a fresh empty one while creating the thread
avoids that specific problem. (Other parameters could cause similar
problems, but solving the namespace one works well enough for now.)

racket/collects/db/private/generic/prepared.rkt

@@ -102,6 +102,9 @@
 (define will-executor (make-will-executor))
 
 (define finalizer-thread
+  ;; Set parameters for the new thread to avoid references to the
+  ;; namespace, since prepared statements may be bound to globals, etc.
+  (parameterize ([current-namespace (make-empty-namespace)])
     (thread/suspend-to-kill
      (lambda ()
        (let loop ()
@@ -116,4 +119,4 @@
                          "prepared statement finalizer thread handled non-exception"])
                   e))])
            (will-execute will-executor))
-       (loop)))))
+         (loop))))))

racket/collects/db/private/sqlite3/connection.rkt

@@ -374,13 +374,13 @@
       (handle-status* who full-s -db db-spec pst))
 
     ;; ----
-
     (super-new)
-    (register-finalizer-and-custodian-shutdown this
-                        (lambda (obj)
+    (register-finalizer-and-custodian-shutdown
+     this
      ;; Keep a reference to the class to keep all FFI callout objects
      ;; (eg, sqlite3_close) used by its methods from being finalized.
      (let ([dont-gc this%])
+       (lambda (obj)
          (send obj disconnect)
          ;; Dummy result to prevent reference from being optimized away
          dont-gc)))))