nvme: Do not rearm timeout for commands without one.

Admin queues almost always have several ASYNC_EVENT_REQUEST outstanding.
They have no timeouts, but their presence in qpair->outstanding_tr caused
useless timeout callout rearming twice a second.

While there, relax timeout callout period from 0.5s to 0.5-1s to improve
aggregation. Command timeouts are measured in seconds, so we don't need
to be precise here.

Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33781

(cherry picked from commit b3c9b6060f9a3525196867d8e812b24fc0bc61e1)