support multiple threads for polldb

fixed pickling error in python 3.8+

change REDIS_CHUNK_SIZE to REDIS_READ_CHUNK_SIZE

updated README

bumped up version

Feature/add range types (toluaina#203)

* add range types

* add range types to test

* add missing type "search_as_you_type" to tests

bumped up version

fixed ModuleNotFoundError

fixed import error and support auth plugins

add support for checkpoint path