rr 4.2.0

[MBot]

Chaos mode added to catch hard-to-reproduce bugs.
Added support for using kernel ftrace while debugging rr itself.
Rework logging code so that logging can be turned on via RR_LOG environment variable or dynamically.
Reworked robust-futex handling.
Work around RDRAND usage in libstdc++3.
Syscall support expanded.
Many bugs fixed, particularly related to Ubuntu 14.04 and 15.10.

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