Represent timeouts using time.Duration

Representing these constants in time.Duration form is more semantic, and allows rewriting

if time.Since(start).Seconds() > myTimeout {

as

if time.Since(start) > myTimeout {

One side effect is that Printf formatting renders a time.Duration as a string like 10s or 100ms, which changes some log messages slightly. If we need compatibility of log message formats, we could use .Seconds() in those places.