handle rate limits (429 status codes) when calling the Stacks Blockchain API

[rafaelcr]

The registrar needs to update the transaction status for a large number of transactions during operation. However, it currently issues all of these calls to the Stacks Blockchain API in a foreach loop which quickly results in 429 status code responses (rate limits).

The registrar should stop sending requests to the API when a 429 is received and resume them later.