Added a conduit call to update arc unit results for a postponed test.

Summary:
It was not possible before to update arc unit results for a postponed
test. This change makes it possible. Also the number of postponed tests
are shown in differential.

Let me know if this looks too Facebook specific.

Test Plan:
Tested the conduit call manually from Conduit Console and updated test
results for a diff that had 20 postponed tests.

Reviewed By: jungejason
Reviewers: epriestley, jungejason
Commenters: epriestley
CC: slawekbiel, aran, tuomaspelkonen, jungejason, epriestley
Differential Revision: 416