[Mondrian] Change 12954: Segment.java synchronization changes finally in

[Eric McDermid]

It's been a while coming, but I've finally checked in the long- 
promised synchronization changes to Segment.java that improve  
concurrency.  We've been using it in production at Marin for months  
with good results, so hopefully the rest of you will see the same  
benefit.

Kudos to Julian for working out the retroweaver 2.0.7 issue that's  
been on my backlog for what seems like forever, which in turn allowed  
me to check this in.

  -- Eric