Refactor keytime:
* Key metadata is kept in a CWallet::mapKeyMetadata (std::map<CKeyId,CKeyMetadata>).
* When generating a new key, time is put in that map, and new key is written.
* AddKeyPubKey and AddCryptedKey do not take a creation time argument, but instead
pull it from that map, if it exists there.
Bugfix:
* AddKeyPubKey and AddCryptedKey in CWallet didn't override the CKeyStore
definition anymore. This is fixed, as they no longed need the nCreationTime
argument now.
Also a few related other changes:
* Metadata can be overwritten.
* Only GenerateNewKey calls GetTime(), as it's the only place where we know for
sure a key was not constructed earlier.
* When the nTimeFirstKey is known to be inaccurate, it is set to the value 1
(instead of 0, which would mean unknown).
* Use CPubKey instead of std::vector<unsigned char> where possible.
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