Impossible to re-register a previously whitelisted candidate

[enlight]

Once a whitelisted candidate is unwhitelisted & unregistered it's impossible to whitelist & re-register them again, the only way to get them back into the candidate list is to allow the DPOS contract to deduct the 1.25M LOOM registration requirement.

Reproduction steps:

1. loom dpos3 whitelist-candidate A
2. loom dpos3 register-candidate A
3. Wait a couple of election cycles.
4. loom dpos3 remove-whitelisted-candidate A
5. loom dpos3 unregister-candidate A
6. Wait a couple of election cycles.
7. Now try re-registering A:
   • loom dpos3 whitelist-candidate A - nope, Cannot whitelist an already whitelisted candidate.
   • loom dpos3 change-whitelist-info A - no error, but doesn't actually do anything since this only works on registered candidates.
   • loom dpos3 register-candidate A - attempts to transfer 1.25M LOOM to DPOS contract since the whitelist amount is still zero.

[enlight]

We should fix a few things here:

• DPOS.WhitelistCandidate should check that the candidate statistic has a zero WhitelistAmount & UpdateWhitelistAmount to determine if a candidate address is whitelisted rather than simply checking if the statistic is nil (because it won't be if the candidate is being re-whitelisted).
• DPOS.RemoveWhitelistedCandidate should also check for a zero WhitelistAmount & UpdateWhitelistAmount to determine if a candidate address is whitelisted.
• DPOS.ChangeWhitelistInfo should do the same thing WhitelistCandidate and RemoveWhitelistedCandidate do to check if a candidate address is whitelist or not.