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Adds a simple "/lag <seconds>" command for anyone with the "server"
priv. When set to anything above zero, no server step will be allowed
to complete in less than that amount of time (it effectively set the
minimum lag per step). CPU cycles will be wasted to meet the lag
requirement (it's a busy-wait).
This is useful for quickly testing lag-dependent issues such as machines
jamming under high lag conditions.
Lag is not preserved across server restarts and is reset to zero.
There is a maximum safe lag level assigned at runtime, which can be
overridden by a setting, but changing the setting requires a restart.
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