Maid-304_trim_pmid_manager_account

The changes are based on following assumptions :
1, DataManager will hold the size info for each chunk
2, PmidNode will not asking for list of storage chunks
3, Double store (different DM choose same PmidNode to store a same chunk) will not happen
4, PmidManager not responsible for notifying DM when a pmid_node becoming online / offline