Make DataNucleus use existing connection for value generation

DataNucleus by default uses a new connection for value generation, so for one thread it might take two connections at a time. That causes a problem when there are many concurrent requests coming in HMS. If all threads are waiting for a new connection, then no threads can proceed.

We can set  datanucleus.valuegeneration.transactionAttribute to "EXISTING" so that DataNucleus will take existing connection rather than request a new one from pool.

Ref:
https://www.datanucleus.org/products/accessplatform/jdo/persistence.html#pmf_properties