Add several functions

1)RecordBasedFileManager::scan: scan returns an iterator to allow the
caller to go through the results one by one.
2)RBFM_ScanIterator::getNextRecord: getNextRecord allow the
RBFM_ScanIterator to get one filtered record one by one.
3)RBFM_ScanIterator::compareValue &
RBFM_ScanIterator::compareWithoutValue:select the rids in the file
which satisfy the specific conditionAttribute and operations( like
equal, not equal or so), and compareWithoutValue is for the condition
with no conditionAttribute.
4)RBFM_ScanIterator.close(): close one iterator for the file and clear
the containers.