improve mcdb_findtagnext() performance
improve mcdb_findtagnext() performance

(thx @jitheshtr for patch; squashed and commit message by @gstrauss)

remove __builtin_prefetch() in mcdb_findtagnext()
remove __builtin_expect() in mcdb_findtagnext()

Removing prefetch improves performance by reducing use of limited
memory bandwidth, especially upon mcdb key lookup misses.  Removing
the branch prediction hint, which was tuned for mcdb key lookup hits,
allows the branch predictor to dynamically predict the branch, which
can be beneficial for use cases such as when majority of mcdb key
lookups are misses.

Performance tests on laptops should note CPU power management settings.
On Linux:
To display CPU power management settings
$ cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
To change CPU power management settings (until next reboot)
$ echo performance | \
  tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

github: closes #8

gstrauss/mcdb,gstrauss/mcdb,gstrauss/mcdb,gstrauss/mcdb,gstrauss/mcdb