refactor mailbox storage and add interfaces to bramble/api

.../org/briarproject/bramble/api/mailbox/MailboxSession.java

.../org/briarproject/bramble/api/mailbox/MailboxStorage.java

...riarproject/bramble/api/mailbox/MailboxStorageStream.java

.../briarproject/bramble/mailbox/AbstractMailboxSession.java

...g/briarproject/bramble/mailbox/MailboxContactSession.java

.../org/briarproject/bramble/mailbox/MailboxManagerImpl.java

...org/briarproject/bramble/mailbox/MailboxOwnerSession.java

...g/briarproject/bramble/mailbox/MailboxSessionFactory.java

...iarproject/bramble/mailbox/MailboxSessionFactoryImpl.java

...arproject/bramble/mailbox/storage/MailboxStorageImpl.java

...ect/bramble/mailbox/storage/MailboxStorageStreamImpl.java