txpool management API's implemented (web3-eth-txpool)

[princesinha19]

Description

I implemented all txpool management API's provided by geth.
https://github.com/ethereum/go-ethereum/wiki/Management-APIs#list-of-management-apis

Fixes partially #2637

Type of change

• New feature

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no warnings.
• I have updated or added types for all modules I've changed
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run test in the root folder with success and extended the tests if necessary.
• I ran npm run build in the root folder and tested it in the browser and with node.
• I ran npm run dtslint in the root folder and tested that all my types are correct
• I have tested my code on an ethereum test network.

[coveralls]

Coverage increased (+0.05%) to 95.428% when pulling b4e43ee on princesinha19:txpool-methods into 839283a on ethereum:1.0.

[coveralls]

Coverage increased (+0.05%) to 95.491% when pulling 32ab9de on princesinha19:txpool-methods into 5e4724b on ethereum:1.0.

[joshstevens19]

If this has to get to through web3.eth you have to add this to the Eth typings class or it wont be exposed in the global scope when ts users try to use it.

https://github.com/ethereum/web3.js/blob/1.0/packages/web3-eth/types/index.d.ts

Please add it

[princesinha19]

@joshstevens19 Done changes.

[princesinha19]

@joshstevens19 thanks for your suggestions. So, I removed this module to get exposed through web3.eth. So, can you please review the new commit?