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

It is also need to be attentioned that in the commit of "take pmid_name out of PmidManagerMetaData" (https://github.com/maqi/MaidSafe-Vault/commit/6e7e2ffb958ddfd50a3e3d49a32b1e41f27b56f6),
pmid_name has been taken out of PmidManagerMetaData which will affect other persona, reflected the changes of MaidManager part.
If it is considered as unsuitable, this commit shall be reverted individually, and have an individual PmidManagerDBMetaData class to be used in PmidManger to save database space and reduce message size.

The committed changes passed compilation of maidsafe_vault.
However, not tried with test_vault or vault compilation.
Neither any test has been undertaken, as a review is preferred first and also works from new account_transfer is required.
It will depend on the method that to be expected in account_transfer or others that accept persona::Key .
at least the previous account_transfer requires a method only available from vault::Key, but not available from PmidName.
Unless we have a consistent design in account_transfer and persona::Key, then the vault::Key can be removed.

But have a separate task to cover this if decided to do so.