Fix EZP-22961: Change solr search handler to use SoftCommit by default

Given we only support Solr 4.x it make sesne to use softCommit by default as
autoCommit takes care about the hard commit every 15 seconds.
Merge pull request #884 from ezsystems/soft_commits

Fix EZP-22961: Change solr search handler to use SoftCommit by default