DashRPC - refactor for browser use

[coolaj86]

This was refactored in preparation for refactoring DashJoin for browser use.

See description in https://github.com/dashhive/DashRPC.js/pull/12.

[coolaj86]

@jojobyte Claiming this task.

Address: Xxp44dxfwwrSeyQbSyD8gETfK2fjKoEa7s

Deliverables:

• See linked pull requests
• https://www.npmjs.com/package/dashrpc/v/20.0.0
• https://github.com/dashhive/DashRPC.js/blob/v20.0.0/README.md
• https://github.com/dashhive/DashRPC.js/issues/13

This shows that the RPC call was done in the browser (using the proxy described in https://github.com/dashhive/DashRPC.js/issues/13) and retrieved a result from the RegTest network:

[jojobyte]

Approved @coolaj86