Add API to query account sequence number in mempool

[zemyblue]

Summary

In order to know the appropriate the sequence number of the account, we need to be able to inquire the sequence number of the account registered in the mempool.

Problem Definition

If some target node is down, we need to wait or change the target node.
However, if the transaction created by the account exists in the mempool in the changed target node when changing the target node, the appropriate sequence number of the account cannot be retrieved.

Background

• The normal query api for the account info is inquired from deliverState.
• This deliverState is updated when the block is confirmed. So we cannot query the information to be reflected in mempool through the normal query api.

So we need to be able to query the sequence number of the account in the mempool.

Proposal

Add the query api about account info from checkState.

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For Admin Use

• Not duplicate issue
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