Adding support for fetching staging logs

Adding ability to fetch staging logs which is especially important when using a custom buildpack.

Details on the changes:
Since CC streams them using keep-alive, and some logs may time long time to be obtained, to avoid socket read timeouts, the read timeout is increased to 5 mins for staging logs.
Attempts are made to follow the offset if ever staging logs would be large and chunked by CC. Might create parasite 404 warn traces.