Add PhasedUpdates mode

[kdnilsen]

In this mode, transactions never have to wait on updates to shared data base. Instead, a new background thread repeatedly rebuilds the data bases from change logs, then replaces the data base using synchronized to assure atomic and coherent replacement. At the start of each transaction, customer threads use synchronized to obtain the most current copy of the data bases. This mode allows simulation of very high allocation rates with very minimal interference from contention locks between reader and writer threads.

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