Get fixed gas price value from tx pool

[kjhman21]

Proposed changes

PROBLEM

1. We have two conflict. GasPrice() APIs in klay_ namespace (PublicKlayAPI and GovernanceKlayAPI)
2. PublicKlayAPI returns gasPrice that is set when the process starts.
3. GovernanceKlayAPI has possibility that it returns wrong value for the last epoch.

SOLUTION

• Remove GasPrice() from GovernanceKlayAPI.
• Update GasPrice() in PublicKlayAPI to get the value from txpool.
• This is draft implementation. We need to check this resolves the issue first.

Types of changes

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• Bugfix
• New feature or enhancement
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Related issues

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Further comments

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