[caffe2][publish] Optimize metanetdef load
Summary:
When loading optional blobs from a large file to workspace, for instance: https://fburl.com/diffusion/l0mcnofg, we are currently loading the file in multiple times. https://fburl.com/diffusion/qhbpyq0e
This diff optimized the load time by loading in the large model file only once, and using the allow_incomplete arg into LoadOp. The implementation of the LoadOp with this arg previously did not delete the blobs that were not found, which is also fixed in this diff.
Test Plan:
Existing unit tests:
```
buck test //caffe2/caffe2/fb/distribute/tests:meta_net_def_storage_utils_test
```
Many sandcastle integration tests.
scuba logs: https://fburl.com/scuba/dai_modelstore/txdf3pjt
Reviewed By: TailofJune
Differential Revision: D27575622
fbshipit-source-id: 7c2b25ef603a378e87ebdbe349c94c2f1952493c
