Pyexasol

- Re-throw `BrokenPipeError` (and other sub-classes of `ConnectionError`) as `ExaCommunicationError`. This type of errors might not be handled in WebSocket client library in certain cases.

- Added optional [`disconnect`](https://github.com/exasol/websocket-api/blob/master/WebsocketAPI.md#disconnect-closes-a-connection-to-exasol) command executed during `.close()`. It is now enabled by default , but can be disabled with explicit `.close(disconnect=False)` to revert to original behaviour;
- Added `csv_cols` to [HTTP transport parameters](/docs/HTTP_TRANSPORT.mdparameters). It allows to skip some columns in CSV and adjust numeric and date format during IMPORT and EXPORT. It is still recommended to implement your own data transformation layer, since `csv_cols` capabilities are limited;

- Added `.meta` sub-set of functions to execute lock-free meta data requests using `/*snapshot execution*/` SQL hint;
- Deprecated some `.ext` functions executing queries similar to `.meta` (code remains in place for compatibility);
- Added connection option `connection_timeout` in addition to existing option `socket_timeout`. `Connection_timeout` is applied during initial connection only and `socket_timeout` is applied for all other requests, including actual login procedure.
- Reworked error handling for HTTP transport to take care of even more complex failure scenarios;
- Reworked internals of SQL builder for IMPORT / EXPORT parameters;
- `ExaStatement` should now properly release result set handle after fetching and object termination;
- Removed `weakref`, it was not related to previous garbage collector problems;
- Renamed previously added `.connection_time` to `.login_time`, which is more accurate name for this timer;
- Query text length in `ExaQueryError` exception is now limited to 20k characters to prevent logs from bloating;
- Fixed `open_schema` function with `quote_ident=True`;
- `.last_statement()` now always returns last `ExaStatement` executed on this connection. Previously it was skipping technical queries from `ExaExtension` (.ext);

ExaConnection

- Added option `client_os_username` to specify custom client OS username. Previously username was always detected automatically with `getpass.getuser()`, but it might not work in some environments, like OpenShift.

ExaConnection

- Added `.connection_time` property to measure execution time of two login requests required to establish connection.

ExaConnection

- Reworked `close()` method. It is now sending basic `OP_CLOSE` WebSocket frame instead of `disconnect` command.
- Method `close()` is now called implicitly during destruction of `ExaConnection` object to terminate IDLE session and free resources on Exasol server side immediately.
- `ExaFormatter`, `ExaExtension`, `ExaLogger` objects now have [weak reference](https://docs.python.org/3/library/weakref.html) to the main `ExaConnection` object. It helps to prevent circular reference problem which stopped `ExaConnection` object from being processed by Python garbage collector.
- Connection will be closed automatically after receiving `WebSocketException` and raising `ExaCommunicationError`. It should prevent connection from being stuck in invalid state.