old structure of LedgerDirsManager brings a lot of usage problem: 1. LedgerDirsManager is not equal to other structures (for example: DefaultEntryLogger ) and cannot be 1-to-1 2. so we need to split into multiple LedgerDirsManager with only one dir
this bring tedious conversion, and it is easy for developers to forget, resulting in conversion problems, so I want to modify the structure of LedgerDirsManager: 1. change it to LedgerDirManager(just only corresponds to one dir) 2. when more than one is needed, use list to store
