Implement DPoS module API

[mehmetegemen]

Description

Implement DPoSAPI class in api.ts and methods

isNameAvailable(name)
getVoter(address)
getDelegate(address)

in it as they are in the specification(https://research.lisk.com/t/define-state-and-state-transitions-of-dpos-module/320#protocol-logic-for-other-modules-75)

getVoter(address), getDelegate(address), isNameAvailable(name) require access to the voter, delegate and name substores respectively, so the first parameter must be apiContext: APIContext to access those substores.

Acceptance Criteria

• All functions are implemented as in the specification
• All functions are fully unit tested