split vmareas on 1st client-requested flush so later flushes are cheaper

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on January 07, 2011 19:49:25_

tools that perform adaptive instrumentation and want to flush fragments periodically to re-instrument are not served will by DR flushing the entire module's worth of fragments: DR should split the vmareas up on the 1st requested flush so later flushes are more performant.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=373_