the JDBC connector should use PDX instances instead of JSON for composite keys

As a GemFire user with composite keys
I want to to replace JSON with PDX for these keys
so that I can support any order for the keys and have minimal memory impact.

Acceptance Criteria:
Given a jdbc-mapping uses a composite key
When I do CRUD operations on data in the region
Then I must specify the key as a "stable" PDX instance that contains fields that match those specified by --id.

Given an existing cache with a jdbc-mapping that uses a composite key
When I want to specify a composite key for a CRUD operation
Then I have a method on the cache to create a "stable" PDX instance.

Given an existing cache with a jdbc-mapping that uses a composite key
When I want to specify a composite key for a CRUD operation
Then I have a method on the cache to create a "stable" PDX instance and the order of fields in the composite key doesn't matter.

Given an existing cache with pdx-read-serialized is set to false
When I do an operation that triggers deserialization of a "stable" PDX instance
Then the "stable" PDX instance will be deserialized as the same "stable" PDX instance.

Given a "stable" PDX instance
When I call its getObject method
Then it will return the same "stable" PDX instance.