Removed old singleton session. Refs #9084.
Return session from login method. Refs #9084.

- Removed all references to old session class.
- Updated catalogs to use new session class.
- Added setter to Session class to clear it when logging out.
Pass parameters by reference in CatalogManager. Refs #9084.

- Updated use of DLLExport to use MANTID_API_DLL.