ovsdb: integrate perf-counter infrastructure into ovsdb-server

ovsdb: integrate perf-counter infrastructure into ovsdb-server

This integration also adds two commands:

ovsdb-server/perf-counters-show -- show all counters
ovsdb-server/perf-counters-clear -- clear all counters

There is no pre-configured sample points. A programmer needs to
added sampling point by changing the source code. However he
does not need to worry about infrastructures such as initialization
or cleaning up memory when ovsdb-server exits.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>