Fix anticheat resetting client position after the client is teleported

Fix anticheat resetting client position after the client is teleported

Previously, m_move_pool could accomodate the client moving from the new
position to the old one, and the server accepted the client to go back
to its old position. However, it couldn't then accomodate the client
moving from its old to its new position, and therefore would reset position
to the old position. Thus, by emptying m_move_pool after a teleport, the
server no longer accepts the client to go back to its old position. A
drawback is however that a laggy client *will* trigger a few
"moved_too_fast" anticheats before being told about its new position.

Don't report player cheated if caused by lag.

Fixes #5118