Update sphinxseach to 2.2.6.

- fixed #2104, ALL()/ANY()/INDEXOF() support for distributed indexes
- fixed #2102, show agent status misses warnings from agents
- fixed #2100, crash of indexer while loading stopwords with tokenizer plugin
- fixed #2098, arbitrary JSON subkeys and IS NULL for distributed indexes
- fixed #2097, escaping of field-start modifier
- fixed possible memory leak in plugin creation function

(fhajny)