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'
* Switched from public fields to private + getters/setters
* Added Postgres profile