sequential inventory syncs with fault tolerance

Add sync tracking for partial inventory syncs (status stored in DB), like full inventory syncs already had.
Fix https://github.com/wayfair-contribs/plentymarkets-plugin/issues/119 by having InventoryUpdateService upgrade partial inventory syncs to full inventory syncs until a full inventory sync is successful.
Make partial inventory syncs consider all variations that have changed since the last successful synchronization (full or partial), instead of doing a blind lookback of a set amount of minutes - fixes https://github.com/wayfair-contribs/plentymarkets-plugin/issues/86
Prevent any inventory sync from starting whilst another inventory sync is in progress
Adjust timeouts for "sync in progress too long, so start a new one"
Before syncing a page of inventory, check if another sync has taken precedence (eg: a full sync started whilst a partial sync is running - partial sync should shut down to let full sync have priority).
Removal of old, dead / useless code
Improvements to logging for inventory syncs

wayfair-contribs / plentymarkets-plugin