Slow down to chunk-wise retrieval of SDR as needed
Slow down to chunk-wise retrieval of SDR as needed

Some BMCs cannot fetch whole SDRs in one chunk.  When faced
with such a scenario, back off exponentially until things can work.

Change-Id: Ifd9df93af56e6fedfeb4d46b662937bf8db80b01

stackforge/pyghmi,openstack/pyghmi,openstack/pyghmi,benoit-canet/pyghmi