ISPN-4061 JPA Cache Store fails on Oracle, Postgres and MsSQL

* MetadataEntity shouldn't use byte[] field as a primary key - as primary key is used SHA-256 hash of the key
* MetadataEntity table name needs to be quoted (tables starting with underscore are not allowed on Oracle, unless quoted)
* Table name 'User' is reserved table on MsSQL - switched to table name 'Users'
* Added Postgres profile
* Switch to private filed in test entities