common: increase default IB ack timer from 16 to 20

common: increase default IB ack timer from 16 to 20

For larger, more congested fabrics, a larger ACK timer is needed.
Consumers can still change default with environment variable
DAPL_ACK_TIMER if they need to increase or decrease.

This applies to SCM and UCM providers only. The CMA provider, which
uses rdma_cm, has no way to control ack timer with current API.

Signed-off-by: Arlin Davis <arlin.r.davis@intel.com>