rename and clean up staking vuex modules

[jbibla]

it's not immediately obvious what these different modules are for or how they relate. i would propose renaming them and ensuring that the separation is necessary. this refactor should also include removing the concept of candidates from the code.

staking.js
validators.js
keybase.js // needs refactor

[faboweb]

as far as I know:
delegates -> all validators, would rename to validators
delegation -> staking related information, would rename to staking
validators -> current validator set, could be merged with delegates

[fedekunze]

the candidates.js module is very time inefficient. There're lots of duplicated/unnecessary for loops. Moving this to high priority as it's making the fee distribution vuex module more complex to implement

[stale]

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