ovsdb: Use DB load time, not on-disk commit times, for compaction.

ovsdb: Use DB load time, not on-disk commit times, for compaction.

The ovsdb-server compaction timing logic is written assuming monotonic
time at milliscond resolution but it calculated the next compaction time
based on the oldest commit in the database. This was a problem because
commit timestamps are written in wall-clock time to second resolution.

This commit calculates the next compaction time based on the time when
the database was first loaded or the last compaction was done, both in
monotonic time at millisecond resolution.

Signed-off-by: Paul Ingram <pingram@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>