Store-gateway overwrites the lazy-loaded.json snapshot while it's still loading blocks at startup

[pracucci]

Today we experienced an issue where we observed store-gateway not correctly loading previously loaded blocks at startup. The issue happens when the store-gateway gets restarted while starting up, and is caused by the fact that lazy-loaded.json is overwritten with the currently loaded blocks while the store-gateway is still startup up.

For example, look at the following sequence of events:

• T0: 100 blocks loaded in the store-gateway and written to lazy-loaded.json
• T1: store-gateway is terminated
• T2: store-gateway is starting, and progressively loading the 100 blocks
• T3: store-gateway has loaded 50 out of 100 blocks, and lazy-loaded.json is updated with the 50 blocks loaded so far
• T4: store-gateway is restarted again, while it was still loading the 100 blocks found in lazy-loaded.json at startup. At this point lazy-loaded.json contains the 50 blocks written at T3, so it lost track of the initial 100 blocks

What we want: we shouldn't update lazy-loaded.json until the initial store-gateway sync has been completed.