Fix garbled JSON-lines output sometimes

[craigds]

Description

In kart diff -o json-lines --add-feature-count-estimate={value}, two threads would write to the output. This correctly used a lock to prevent races, but there is an intermediate bytearray which is written just beforehand (introduced with #1025) and this also needed to use a lock to prevent races.

This change moves that intermediate bytearray under the same lock as writing the output buffer, thus avoiding the race condition

Related links:

Checklist:

• Have you reviewed your own change?
• Have you included test(s)?
• Have you updated the changelog?

[craigds]

example of output

$ kart diff '42f601b^?...42f601b' -o json-lines --no-sort-keys --add-feature-count-estimate=exact
...
{"type":"feature","dataset":"nz_property_titles","change":{"+":{"fid":95,"geom":"01060000000100000001030000000100000009000000ACCE0CD2D8D66540F81CC19B6B6542C0378D3241DAD665408EEA74206B6542C050D6FE3EDAD66540D1A55AF06B6542C0E401D63ADAD6654036DEF8BF6C6542C0712BB834DAD66540DE10088F6D6542C0F520AE2CDAD66540E329645D6E6542C02C1725FDD9D665402732049B726542C0ACCE0CD2D8D6654045337118726542C0ACCE0CD2D8D66540F81CC19B6B6542C0","id":344  File "/opt/homebrew/Cellar/[email protected]/3.11.11/Frameworks/Python.framework/Versions/3.11/lib/python3.11/threading.py", line 982, in run
4244,"title_no":"NA36D/718","status":"LIVE","type":"Freehold","land_district":"North Auckland","issue_date":"1976-12-24T00:00:00.000","guarantee_status":"Guarantee","estate_description":"Fee Simple, 1/1, Lot 1 Deposited Plan 80161, 610 m2","number_owners":1,"spatial_extents_shared":"F"}}}
    self._target(*self._args, **self._kwargs)
  File "/Users/cdestigter/code/kart/kart/json_diff_writers.py", line 288, in _calculate_and_feature_count_estimate
    self.dump(
  File "/Users/cdestigter/code/kart/kart/json_diff_writers.py", line 252, in dump
    self._output_buffer.extend(b"\n")
BufferError: Existing exports of data: object cannot be re-sized
{"type":"featureCountEstimate","accuracy":"exact","datasets":{"nz_property_titles":98}}000010000000103000000010000000D0000004A65E5F2FAD66540E15A23C2636542C0730475B5F9D66540D0EF8C51566542C043131234F8D665400376DABF576542C05514AFB2F6D665405D42E02D596542C0FACA88F6F5D665405117F333516542C09BCC18A0F5D6654009C2A6694F6542C0A8E282CBF6D665402C43084D4E6542C0013B81F5FAD665402C43084D4E6542C0DE9A8A89FBD6654091282393546542C0E657B80BFAD66540777915FE556542C0658A3908FAD66540A231B801566542C00E1C8645FBD6654070A2DD72636542C04A65E5F2FAD66540E15A23C2636542C0","id":4410549,"title_no":"330406","status":"LIVE","type":"Cross lease","land_district":"North Auckland","issue_date":"2007-05-17T15:37:29.000","guarantee_status":"Guarantee","estate_description":"Fee Simple, 1/2, Lot 26 Deposited Plan 40676, 2,102 m2\r\nLeasehold, 1/1, Flat 1 Deposited Plan 382696","number_owners":2,"spatial_extents_shared":"T"}}}