Flask-signalbus

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- Do not wait after the first DB serialization failure
- Pinned `pytest`, `pytest-cov`, `mock` versions
- Improved docs

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- Changed a message loglevel from "warning" to "info".

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- Added support for `send_signalbus_messages` class method. This can
greatly improve performance by receiving acknowledges for a whole
batch of messages at once.
- Added `flushordered()` method, and `flushordered` CLI command. This
allows sending messages in predictable order, and when auto-flushing
is disabled -- the correct order is guaranteed.
- General code improvements.
- Improved documentation.

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- Add `models` parameter to flushmany().
- Do not order by primary key if `signalbus_order_by` is not set.

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- Improved performace.
- Added support for `signalbus_order_by` attribute on signals.

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- Added `*options` parameter to `_ModelUtilitiesMixin.get_instance()`
and `_ModelUtilitiesMixin.lock_instance()`.