MINOR: haload: keep old-scheduled users under -ee's floor rate

MINOR: haload: keep old-scheduled users under -ee's floor rate

A user with its next send already scheduled before -ee starts still
fires at the old, unlimited pace. Nothing re-paces it once the ramp
begins. A mass failure could still cause a burst of new connections
right when the floor kicks in.

Each user now remembers the last -ee generation it checked in with.
On its first wakeup during a new ramp, it checks for room under the
current rate before sending. If there is none, it retries after a
short, fixed delay, and checks again. A user created during the ramp
already has the right generation, so replacements are not affected:
-ee's doubling still only depends on their own successes.

This check does not reuse hld_usr_schedule()'s wait: that one grows
with the number of idle users on the thread, and right after a mass
failure that number can be huge. It also never gets recomputed later,
so a user could end up waiting several minutes with no way back. A
short, fixed retry avoids this, since the user is checked again each
time instead of trusting one old estimate.