Create prioritization for signature requests

[ChaoticTempest]

Currently, we just take arbitrary sign requests in our sign request pool, but not exactly based on any sort of metric. So we may end up deprioritizing certain requests over the other. So we should create a prioritization scheme such that we do not potentially starve any requests. We can base it on whenever the request entered the system or on a payment in the form of a NEAR deposit to put the request ahead of the queue.

Related tickets

• Paid requests #583
• Potential DoS of main functionality by keeping the queue full #576