prefer const string& over char* in CDB and CWalletDB constructor

- also make parameter of CDBEnv::CheckpointLSN a constant reference
cleanup class private and public areas in walletdb

- only code movement
Merge pull request #4931

93f84d0 cleanup class private and public areas in walletdb (Philip Kaufmann)
22d7e70 prefer const string& over char* in CDB and CWalletDB constructor (Philip Kaufmann)