JVM Snapshot Implementation

[gegao]

I implement a feature that enables H-Store to execute OLAP queries on JVM Snapshot to avoid expensive distributed transactions. The snapshots are created using fork() system call which has Copy-On-Write mechanism.

Main changes include:

• fork() and kill() JNI.
• JVM Snapshot Manager.Main part of the code. It runs in a separate thread in the H-Store site. The Manager is in charge of creating, refreshing snapshots, communicating with snapshots and queuing OLAP queries.
• JUnit test
• Some changes to other parts of system includes snapshot initialization, snapshot synchronization etc.

This feature is under flag site.jvmsnapshot_enable. Details can be found in http://hstore.cs.brown.edu/documentation/deployment/jvm-snapshots/