ovsdb: Compact databases online automatically and on-demand.

ovsdb: Compact databases online automatically and on-demand.

If the database grows fairly large, and we've written a fair number of
transactions to it, and it's been a while since the database was compacted,
then (after the next commit) compact the database.

Also, compact the database online if the "ovsdb-server/compact" command is
issued via unixctl.  I suspect that this feature will rarely if ever be
used in practice, but it's easier to test than compacting automatically.

Bug #2391.