Add an Index field

Index represents the position of an entry within its parent. The start
entry should be index 0, and every entry afterward should increment by
one. JournalSummary and JournalUpdate also have a Count value in
addition. Their Index is the first index out of the list of entries they
summarize, and the count is the number of entries. Together, this lets
clients figure out whether they have all of a journal's data.

Also, LogUniverse now does a Flush() operation on disposal.