Move background table loading code in to a TableLoadingMgr class

Moved some code from CatalogServerCatalog in to its own TableLoadinMgr class.

Change-Id: Iceb2f1086b2689d437ddfa3ebfcc057fe3eeccd8
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1571
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins