add log to denote current storage manager name.
delete unused classes
Override equals and hashCode method at Block.
add parameter about buffer configuration
change code format (try-catch clause)
add buffer replace policy interface
add LRU buffer module. its implement ReplacePolicy interface.
changed return value of buffer manager interface
add buffer manager module and its test code.
add MAID storage manager test code.
add test suite for refactoring ykstorage