Use actual bytesarray/memoryview instead of bytes concatenation

Bytes concatenation such as this is extremely wasteful and is never a good idea.

https://github.com/fluent/fluent-logger-python/blob/master/fluent/sender.py#L137
```python
# buffering
if self.pendings:
self.pendings += bytes_
bytes_ = self.pendings
```

Python has mutable buffers that should be used to ensure that the objects aren't created needlessly putting pressure on GC and wasting CPU with double-copy concatenation.