Crate

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- more reliable failover mechanism

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- improved server failover / retry behaviour

- use bulk_args in executemany to increase performance:
With crate server >= 0.42.0 executemany uses bulk_args
and returns a list of results.
With crate server < 0.42.0 executemany still issues
a request for every parameter and doesn't return
any results.

- improved docs formatting of field lists

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- fix: ``cursor.executemany()`` now correctly sets the cursor description

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- fix: correctly attach server error trace to crate client exceptions

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- fix: only send ``error_trace`` when it is explicitly set

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- expose the ``error_trace`` option to give a full traceback of server exceptions