Research /dev/urandom concurrent access

[bernardnormier]

We currently assume that concurrent reads of /dev/urandom can return the same bytes, which was true years ago but may not longer be true today.

As a result, we synchronize reads to /dev/urandom (within the same process):
https://github.com/zeroc-ice/ice/blob/v3.7.1/cpp/src/IceUtil/Random.cpp#L31

and also replace 15 random bits in UUIDs by our PID, resulting in only 122 - 15 = 107 random bits, see:
https://github.com/zeroc-ice/ice/blob/v3.7.1/cpp/src/IceUtil/UUID.cpp#L131

We could possibly use instead a C++ Random Number Engine: http://en.cppreference.com/w/cpp/numeric/random

ICE-8843

[pepone]

We switched to std::random in #1715