ovsdb: write commit timestamps to millisecond resolution.

ovsdb: write commit timestamps to millisecond resolution.

This is expected to make system debugging easier.

This raises two compatibility issues:
1. When a new ovsdb-tool reads an old database, it will multiply by 1000 any
  timestamp it reads which is less than 1<<31. Since this date corresponds to
  Jan 16 1970 this is unlikely to cause a problem.
2. When an old ovsdb-tool reads a new database, it will interpret the
  millisecond timestamps as seconds and report dates in the far future; the
  time of this commit is reported as the year 45672 (each second since the
  epoch is interpreted as 16 minutes).

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