System performance benchmark

I would like to be able to generate SQL transactions at a steady rate,
independent of the response time of those transactions. This could be used
to examine a system under test driven at a constant load, while altering
the configuration of the database, operating system, hardware, number of
client threads, etc.

I have actually implememented this change, and a patch against sysbench
0.4.12 is attached. This was tested on Solaris, but POSIX interfaces were
chosen for compatibility with at least Linux. I did _not_ implement the
full autoconf trickery to make this foolproof for all platforms.