[development] optimize table in cron

[vlado]

> > > Has it ever been discussed to run a query to optimize some of the more
> > > active tables in MySQL during cron.php runs? For example 'OPTIMIZE
> > > TABLE
> > > watchdog;'
> >
> > OPTIMIZE TABLE is a MySQL-ism; it's not ANSI SQL.
That's true, but from the currently summorted databases postgress carries out
automatic optimisations, mysql accepts OPTIMIZE. 

Can't we make an db_optimize() abstraction function, with mysql backend
using OPTIMIZE TABLE, the default other doing nothing, and if more 
backends are added they will be resposible for doing something else
than nothing.

> This could be added in a conditional structure to dba.module
maybe.
Vlado