Set upper limit on the fee rate

[Keith-CY]

Is your feature request related to a problem? Please describe.
The recommended fee rate returned RPC is purely based on statistical results from on-chain data without any manual fix. This means that its value can become exceptionally high due to abnormal circumstances and lead to a waste of CKB.

Describe the solution you'd like
Set an upper limit on the fee rate for Neuron to prevent any exceptional value from RPC and applying unreasonable value under extreme circumstances.

Additional context
This feature request comes from the community.

[yanguoyu]

nervosnetwork/neuron#3246 Using the median is better than mean