#128 Add parameter to control frequency of database optimisation

[Matt Brown]

Currently PHPwiki has a fairly basic scheme with a
hard-coded parameter for how often to optimise the wiki
backend.

This patch makes that time period configurable and if
it is set to 0 completely disables database optimisation.

This patch is part of the WLUG Wiki merge back into
upstream PHPwiki source. It represents revisions 24:25
in the WLUG svn repository: See

http://svn.wlug.org.nz/viewcvs?root=wiki&view=rev&rev=24
http://svn.wlug.org.nz/viewcvs?root=wiki&view=rev&rev=25

[Matt Brown]

Adds database optimisation frequency parameter