db-ctl-base: make use of user supplied exit function

db-ctl-base: make use of user supplied exit function

The user is required to expose the_idl and the_idl_txn global variables,
so that memory can be cleaned up on fatal errors. This patch changes to
ask user to supply an exit function via ctl_init(). What user needs to
do on exit can now remain private.

Signed-off-by: Andy Zhou <azhou@nicira.com>