MINOR: haload: add -T to add a think time after each response

MINOR: haload: add -T to add a think time after each response

Add -T <time>, like h1load's option of the same name: after a
response, wait <time> ms (with a bit of random jitter) before sending
the next request, instead of sending it right away. This simulates a
real user pausing between actions.

-T is positional, like -post or -head: it applies to the URLs that
follow it on the command line, not to all of them.

If -R is also set, the wait used is the larger of the two: the rate
limit and the think time never fight each other, whichever asks for
more waiting wins.

-ee takes over as soon as an error is detected: -T is ignored while it
is still probing, and applies again once probing is done and the
normal load resumes.