Replace spring.datasource.initialization-order with a JPA-specific property

[wilkinsona]

spring.datasource.initialization-order was introduced in M2 to allow DataSource initialization to be deferred until after any EntityManagerFactory had been created. This meant that the general initialization had become somewhat aware of a JPA specific concept. That's not too bad for JDBC, but when we support initialization with R2DBC as well it'll become a bigger problem.

We should replace spring.datasource.initialization-order with a JPA-specific property.