Expose size and lease expiration information in a single network API

Add something like this:

def stat_shares(storage_index: [StorageIndex]) -> [ShareStat]:
    """
    For each storage index, retrieve the size and latest lease expiration time (or None for missing information).
    """

where ShareStat has the stored size of the shares for the given storage index and the lease expiration times, if there are any leases.

This is required for automatic lease maintenance, #44 (closed). See discussion there for reasoning.