Next()'s result is a tri-state - failed, more to go, complete. Replace
the way that this is returned with an enum with values FAIL, MORE, and
DONE rather than with two booleans.
Adds test coverage for the wallet's crypted key loading from db process.
The following scenarios are covered:
1) "All ckeys checksums valid" test:
Loads an encrypted wallet with all the crypted keys with a valid checksum and
verifies that 'CWallet::Unlock' doesn't force an entire crypted keys re-write.
(we force a complete ckeys re-write if we find any missing crypted key checksum
during the wallet loading process)
2) "Missing checksum in one ckey" test:
Verifies that loading up a wallet with, at least one, 'ckey' with no checksum
triggers a complete re-write of the crypted keys.
3) "Invalid ckey checksum error" test:
Verifies that loading up a ckey with an invalid checksum stops the wallet loading
process with a corruption error.
4) "Invalid ckey pubkey error" test:
Verifies that loading up a ckey with an invalid pubkey stops the wallet loading
process with a corruption error.
