Bug 22705: Change default value of cxn_pool to 'Static'

Bug 22705: Change default value of cxn_pool to 'Static'

'Static' is the default value of Search::Elasticsearch and for a good
reason : it works in most cases, unlike the 'Sniff' option

To test:
1 - Apply patch
2 - Edit koha-conf.xml
3 - Add '<cxn_pool>Static</cxn_pool>' to the elasticsearch stanza
4 - Restart all the things!
5 - Reindex ES, it works
6 - Set SearchEngine to ES, try searching
7 - It works!

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>