Bug 19502: Retrieve index.max_result_window from ES

Bug 19502: Retrieve index.max_result_window from ES

This avoid hardcoding '10000' in two different places and allow users to
adjust this setting.

Also, this patch fixes a bug when the search return less than 10000
results

Test plan:
1. Do a search that returns 10000+ records.
2. Note the warning above the pagination buttons
3. Go to the last page, no error
4. Change the ES setting:
   curl -XPUT http://elasticsearch/koha_master_biblios/_settings -d \
     '{"index": {"max_result_window": 20000}}'
5. Do another search that returns more than 10000 but less than 20000
6. Note that the warning does not show up
7. Go to the last page, still no error

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>