Attempt to work around another ender-pearl + survivalfly issue.

Jump (+run) and throw an ender pearl at a nearby entity. 
Survivalfly might trigger with limitjump, since the MediumLiftOff gets 
reset on teleporting (would appreciate MC adding sequence numbers to 
moving packets :p).
Confine "sprintback" alerts to speeds greater than walking.

This reduces false positives with throwing enderpearls at nearby
entities or for horizontal velocity.

Missing: Use up horizontal velocity if sprintback is triggered and
queued velocity is present.