ovsdb: Use table indexes if available for ovsdb_query().

ovsdb: Use table indexes if available for ovsdb_query().

Currently all OVSDB database queries except for UUID lookups all result
in linear lookups over the entire table, even if an index is present.

This patch modifies ovsdb_query() to attempt an index lookup first, if
possible. If no matching indexes are present then a linear index is
still conducted.

To test this, I set up an ovsdb database with a variable number of rows
and timed the average of how long ovsdb-client took to query a single
row. The first two tests involved a linear scan that didn't match any
rows, so there was no overhead associated with sending or encoding
output. The post-patch linear scan was a worst case scenario where the
table did have an appropriate index but the conditions made its usage
impossible. The indexed lookup test was for a matching row, which did
also include overhead associated with a match. The results are included
in the table below.

Rows                   | 100k | 200k | 300k | 400k | 500k
-----------------------+------+------+------+------+-----
Pre-patch linear scan  |  9ms | 24ms | 37ms | 49ms | 61ms
Post-patch linear scan |  9ms | 24ms | 38ms | 49ms | 61ms
Indexed lookup         |  3ms |  3ms |  3ms |  3ms |  3ms

I also tested the performance of ovsdb_query() by wrapping it in a loop
and measuring the time it took to perform 1000 linear scans on 1, 10,
100k, and 200k rows. This test showed that the new index checking code
did not slow down worst case lookups to a statistically detectable
degree.

Reported-at: https://issues.redhat.com/browse/FDP-590
Signed-off-by: Mike Pattrick <mkp@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>