Reportportal-client

Added
- `__getstate__` and `__setstate__` methods in `RPClient` class to make it possible to pickle it, by HardNorth
Changed
- `token` field of `RPClient` class was renamed to `api_key` to maintain common convention, by HardNorth
Fixed
- Issue [214](https://github.com/reportportal/client-Python/issues/214): HTTP RFC compliance fix for getting project settings, by hanikhan

Added
- Check for parent `RPClient` object in thread before logging, by HardNorth

Added
- `RPClient.clone()` method, by HardNorth
Fixed
- Client crash in case of Client ID reading error, by HardNorth

Fixed
- Client crash in case of Client ID saving error, by HardNorth

Added
- `MAX_LOG_BATCH_SIZE` constant into `log_manager` module, by HardNorth
Fixed
- Missed `verify_ssl` argument passing to `LogManager` class, by rplevka
Changed
- Statistics service was refactored, by HardNorth

Fixed
- Issue [198](https://github.com/reportportal/client-Python/issues/198): Python 3.8+ logging issue, by HardNorth
- Issue [200](https://github.com/reportportal/client-Python/issues/200): max_pool_size not worked without retries setting, by ericscobell
- Issue [202](https://github.com/reportportal/client-Python/issues/202): TypeError on request make, by HardNorth
Changed
- Statistics service rewrite, by HardNorth
Removed
- Deprecated code, `service.py` and `LogManager` in `core` package, by HardNorth