Notification of string changes

Author: sam marshall
MDL-83119 search_solr: Implement check on connectivity, space usage
Implements a status check which confirms that the Solr search engine
is available. Optionally, the check can also show a warning if the
index grows beyond a certain size.

As part of this change, a new API was added in search_solr\engine
to allow using http_client (Guzzle) instead of raw Curl; this makes
it easier to create mock tests in PHPunit for the new functionality.


http://git.moodle.org/gw?p=moodle.git;a=commit;h=b549491bb39826b292f088ceb66cec6bc1c7ca2b
http://github.com/moodle/moodle/commit/b549491bb39826b292f088ceb66cec6bc1c7ca2b

+ 5.0 en [check_indextoobig,search_solr]
+ 5.0 en [check_nocore,search_solr]
+ 5.0 en [check_nosize,search_solr]
+ 5.0 en [check_notconnected,search_solr]
+ 5.0 en [check_time,search_solr]
+ 5.0 en [indexsize,search_solr]
+ 5.0 en [indexsizelimit,search_solr]
+ 5.0 en [indexsizelimit_desc,search_solr]