The current IPMI KCS code is waiting 100us for all transitions (roughly
between each byte either sent or received). However, most transitions
actually complete in 2-3 microseconds.

By polling the status register with a delay of 4us with exponential
backoff, the performance of most IPMI operations is significantly
improved:

• A BMC update on a Supermicro x9 or x11 motherboard goes from ~1 hour to ~6-8 minutes.
• An ipmitool sensor list time improves by a factor of 4.

Testing showed no significant improvements on a modern server by using
a lower delay.

The changes should also generally reduce the total amount of CPU or
I/O bandwidth used for a given IPMI operation.

Submitted by: Loic Prylli <lprylli@netflix.com>
Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20527