Fix MySQL connection problems by making back_log=5000 work by setting net.core.somaxconn=5000

When running tests that accessed MySQL, I was getting errors like:

BENCHMARKING Query ... [MySQL] 2013/05/23 00:57:04 packets.go:31: EOF
2013/05/23 00:57:04 Error scanning world row: driver: bad connection
exit status 1

Debugging this showed that the web server box had more connections to MySQL
than expected (i.e. netstat on the MySQL box showed 100 TCP connections, but
the web server box showed 1200). So there was some sort of TCP problem, perhaps
lost ACKs as described in http://www.evanjones.ca/tcp-stuck-connection-mystery.html .

Looking at the MySQL documentation for back_log (http://dev.mysql.com/doc/refman/5.0/en/server-system-variables.html#sysvar_back_log),
it suggests that the OS setting needs to be greater than or equal to back_log. Thus,
we need to set net.core.somaxconn as big as our back_log value.

I tried this and the connection problems went away.