Trinity.C: MT_SHADOW_ENUMERATOR needs to be polished

[yatli]

https://github.com/Microsoft/GraphEngine/blob/multi_cell_lock/src/Trinity.C/src/Storage/MTHash/MT_SHADOW_ENUMERATOR.cpp#L43

The enumerator reports the status code from local memory storage use cell operation back to LMS enumeration routine, but if someone deleted the cell, the status code should be E_CELL_NOT_FOUND, and will cause the enumeration routine to stop the enumeration on the current trunk and move to the next one before all other entries are enumerated.

Also, https://github.com/Microsoft/GraphEngine/blob/multi_cell_lock/src/Trinity.C/src/Storage/MTHash/MT_SHADOW_ENUMERATOR.cpp#L20

Currently the memory cost of an enumerator grows linear to the data size. This is bad if we have a lot of threads doing enumerations together.