Add debugging code for simulating packet loss in outgoing UDP packets. The
algorithm for dropping packets is a modified two state algorithm for creating
bursts of dropped packets. As configured there is about a 1.5% drop rate.

Invocation of the packet loss code is commented out by default.
Enable runtime configuration of the minimum rate for adaptive
throttles. Setting adaptive_throttle_min_bps will change the
minimum rate that the adapative throttles will drop to in case
of network packet loss. The current rate default rate is 256kbps.
The viewer can throttle rates under that amount, but the dynamic
adaptation will not.
Change the effect of successfully acknowledged packets to bump the
adaptive throttle by a full MTU. This is consistent with some implementations
of congestion control algorithms and certainly has the effect of opening
the throttle window more quickly after errors. This is especially important
after initial scene load when the number and size of packets is small.
Fix the throttle tests. Remove the hardcoded constant multipliers and
compute the expected values without depending on the token bucket code.
Another technique inspired by some of the newer flow control algorithms... rather
than drop exponentially to 0 (and then adjust up for the minimum flow), drop on
the delta between current rate and the minimum rate. This should smooth the fallback
to minimum.
Turn on logging of old acks, this is for debugging only, stats version coming later
Comment out the debugging statements added in the last commit. Keeping
them in the code for later use rather than just reverting them.

Fixed the throttle tests for the new algorithm used when packets
are marked as expired.
Merge branch 'mb-throttle-test'

Merge in the new throttle code.