remove old code from mailbox session

the mailbox

that no empty streams are sent to the mailbox and update session
Constructors

or acks need to be send to a given ContactId

the mailbox
add basic implementation of MailboxStorage (For testing)

without a mailbox

sessions have different requirements for TAKE/STORE requests depending
on the peer's permissions)

with NULL ContactId

registered before reading requests) and cleanup

the tag from the encrypted stream)