Stop generating a fresh JWT for each polling request when signing, and poll with exponential backoff

[diox]

Currently, web-ext sign polls AMO API every second for validation and approval results and each of these requests generates a fresh JWT.

JWT for AMO API have a max expiration time of 5 minutes, and are now documented as being reusable. web-ext should take advantage of this and:

• Re-use the JWT generated at the beginning of the submission process for a few minutes (let's say up to 15 seconds before expiration date to give some leeway)
• Use exponential backoff to avoid polling that much both at validation and approval.