Add options to control dormant-client feature.

The DormantClientTimeout option controls how long Tor will wait before
going dormant.  It also provides a way to disable the feature by setting
DormantClientTimeout to e.g. "50 years".

The DormantTimeoutDisabledByIdleStreams option controls whether open but
inactive streams count as "client activity".  To implement it, I had to
make it so that reading or writing on a client stream *always* counts as
activity.

Closes ticket 28429.