Update _next_unique_id when setting them manually
Update _next_unique_id when setting them manually

This lets us switch back to automatic setting seamlessly, for code that
e.g. does automatic mesh refinement after a manual setting.

jwpeterson/libmesh,jwpeterson/libmesh,libMesh/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,libMesh/libmesh,jwpeterson/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,dschwen/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,libMesh/libmesh,libMesh/libmesh,dschwen/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,dschwen/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,libMesh/libmesh,jwpeterson/libmesh,roystgnr/libmesh,jwpeterson/libmesh,dschwen/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,libMesh/libmesh,roystgnr/libmesh,roystgnr/libmesh,dschwen/libmesh,libMesh/libmesh,roystgnr/libmesh,dschwen/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh