Deprecate RepositoryJobExecutionContext#get*Session and replace with RepositoryJobExecutionContext#create*Session

Since the sessions returned by the RepositoryJobExecutionContext#getSession(Credentials) and RepositoryJobExecutionContext#getSystemSession() methods are newly created sessions that must be logged out by the caller, it is an improvement to name these methods createSession and createSystemSession instead. This way, it is more clear that the sessions must be logged out.