Implement txpool JSON-RPC namespace

[crypto-facs]

Proposal: [Description of the feature]
The absence of this method is already documented here but I am creating the request since it is a required method for an open source solution we are using for the future block explorer of the new chain.

Thanks in advance for your time guys, the current transaction fixes are more important as they are blockers on different areas. If maybe this and/or the development of the other missing methods is something we could be of help (the crypto.org team) please let us know.

Current behavior: [What currently happens]
Non existent

Desired behavior: [What you would like to happen]
The expected behavior of this method is documente here.

Use case: [Why is this important (helps with prioritizing requests)]
This is currently important as third party solutions we are using for the development of the chain eco system expect it. It is currently a blocker for the block explorer development.

Requests may be closed if we're not actively planning to work on them.

[crypto-facs]

Will summarize offline discussions about this issue. Given that the concept of pending transactions works different in Tendermint than Ethereum (more info here), there have been discussions about what the appropriate behavior of this endpoint should be. Two options where discussed:

• Query the mempool of the node and return pending transactions specific to the node. In Tendermint, mempool are node specific pending transactions before every commit, as it offers instant finality (more info here).
• Return an empty response to avoid misleading information for now until a better solution is designed

For now, the decision was to go with the second option to unblock the integration of third party components of the chain ecosystem that require this endpoint to exist.