Fixing GIL issue in message_reflash_callback

[drebbe-intrepid]

This fixes an issue where we don't always re-lock the GIL when flashing devices (mainly through open_device or force_firmware_update) and calling the message callbacks.

Need to cherry-pick 6ab321b pull request #51 into other branches besides master.

[drebbe-intrepid]

@pierreluctg