Bug 17610 - Allow the number of plack workers and max connections to be set in koha...

Bug 17610 - Allow the number of plack workers and max connections to be set in koha-conf.xml

It would be nice if we could control the number of workers and max
requests on a per instance basis, rather than the numbers being
hardcoded in the plack startup script.

Test Plan:
1) Build a new package of Koha with this patch applied ; )
2) Verify koha-plack still works
3) Add the following to the config section of your koha-conf.xml:
 <plack_max_requests>75</plack_max_requests>
 <plack_workers>4</plack_workers>
4) Stop plack
5) Start plack
6) Verify the number of works and max requests worked!

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Larry Baerveldt <larry@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Rebased against master and added a description for the new configuration
entries

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>