Handle appending vectors to a tile that is locked

[robyngit]

When the viz-staging step is run in parallel during the viz-workflow, errors arise when two processes are trying to save polygons to the same tile file. Some of the errors that came up during testing that I believe are related to this issue are:

• fiona.errors.DriverError: database is locked
• fiona._env: failed to execute insert : database is locked
• fiona._env: sqlite3_exec(UPDATE gpkg_ogr_contents SET feature_count = 613 WHERE lower(table_name )= lower('1047')) failed: database is locked
• RuntimeError: GDAL Error: failed to execute insert : database is locked
• fiona._env: sqlite3_exec(DROP TRIGGER "trigger_insert_feature_count_1057") failed: database is locked

The TileStager.save_tiles() method should check if a file is locked for writing and then wait for it to be available before trying to append to it. Alternatively, if a tile already exists, the TileStager could save the tile to a separate tile, then tiles could be merged at the end of staging.

[robyngit]

Used filelock within the TileStager.save_tiles method, which should prevent another process from writing to the same tile at the same time. Tested by re-staging the same files* in parallel that threw the errors listed above. Did not get any "is locked" errors, so I think this solves the problem. Though given that @KastanDay didn't see any of these errors when he ran the code, it's possible that these write conflicts happen rarely. We can re-open this issue if it comes up again.

*Staged: 448 files (24.73 GB) from the /home/pdg/data/ice-wedge-polygon-data/version 01/alaska/207_208_209_223_224_iwp directory on datateam.

[KastanDay]

Awesome. That seems like a more or less ideal solution. I'll do larger testing ASAP (i.e. this week) to see if I can reproduce this bug.