Add authentication caching support

[tsg-]

COSbench currently uses as many authentication threads as number of workers at each stage. Each authentication thread performs identical operation and in return receives the same authentication token. In most cases, this is not needed - one authentication worker is enough to obtain auth token and remaining workers can reuse the same.

[tsg-]

This will build on commits c1de48604d5f6b54f344eef6a8df6294b1275b6f (Perform authentication once and cache token) and 9536e12 (avoid being challenged for each request)

[ywang19]

Fixed through commit 257e975.