Burn DPR to recover the decreased credit

[kernel1983]

Summary

The deeper devices must repeatedly submit ImOnline to keep the heartbeats with a deeper chain.
However, the device may accidentally fail to submit the heartbeat, which leads to the decrease of the credit.

Motivation

To avoid such consequences, we proposed to purchase the credit with DPR.
Users should be able to recover their credit lost if they will.
But the amount of credit cannot exceed the highest credit of the user before.

Design

In the design, we simply added a method in the credit pallet.
The DPRs consumed for the credits are sent to the treasury module (account).

The price for the credit purchased needs to be discussed within the community.
Once the price is adjusted well until we see a pattern, we could automate it in the future version.