Convert local rendevzous table to use WeakPtr.

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Convert local rendevzous table to use WeakPtr.

A new EagerContext stat is added for inspecting number of 'leaked' local Rendezvous objects. I checked this metric manually and with this CL there is no apparently leak from a full train step from save_restore_v2_test.py.

When a rendezvous has no pending requests it will be automatically destroyed. This is achieved by tracking a reference count of the owner rendezevous from the Item member of the LocalRendevevzous.

The global rendezvous is kept alive by the eager context.