Tweak the Journal interfaces

* IJournalReader is now JournalSnapshot, and represents a read-only
  snapshot of a journal, according to the event sourcing framework.

* The members introduced in IJournal are now in their own separate
  interface, IJournalWriter, which is write-only. No information about
  the journal is provided through this interface.

* To effect the last, IWriteOnlyTextChannel was created.
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