ovsdb: Add per transaction commit instruction counter

ovsdb: Add per transaction commit instruction counter

Measure user space only instruction counters for commit each
transaction. This measurement is mainly useful for
tuning OVSDB internal implementation, such as how OVSDB scales over
number of client connections.

A simple usage example:

ovs-appctl -t ovsdb-server ovsdb-server/perf-counters-clear
ovs-vsctl add-port br0 p3
ovs-appctl -t ovsdb-server ovsdb-server/perf-counters-show
ovsdb_txn_commit            2      906922 453461.0

The commands above show that the 'add-port br p3' command caused ovsdb
to execute 2 transactions. Each transaction takes 453461 instructions,
total 906922 instructions. The number will vary based on number of
ovsdb clients connected.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>