improve the delimiter for userId and pool Id for session pools

we will change the default delimiter from @ to '\uFFFF'

By Woonsan:

FYI, You will need to change those in two locations:
(1) BasicPoolingRepository.java:
private String defaultCredentialsUserIDSeparator = "\uFFFF";
(2) SpringComponentManager.properties:
repository.pool.user.name.separator = \uFFFF