Elasticsearch test refactorings

Changes:
 abstract connection behavior to ElasticsearchTestCase
 move _remove and _search methods to ElasticsearchTestCase
 remove index in tearDown to avoid test dependency

Related fixes:
 get_last_doc now runs on empty index correctly with latest ES
 test suite now always connects to the configured ES cluster
Avoid doing scroll queries when only interested in count
Fix test_stress method to actually test docs are inserted
Use the scan helper in _stream_search
Use streaming_bulk to avoid loading huge response lists into memory
Use a filter instead of query