DEBUG: Add logs between fetch data and caching

[aakoshh]

Following the log debug session on Slack I see that these lines can take up to 15 minutes to execute. It could be some freakishly slow API calls or STM live- or deadlocking.

The PR adds some logging in between these.

Suspicious logs:

"2024-02-27T19:44:01.278908Z", "message":"syncing heights","chain_head":1391238,"pointers":"{tail: {height: 1391199, hash: 9bdf3eae78d33514b3cf262cc16eca91a75845361a906606b660afadbba72390}, head: 1391199}"},"filename":"fendermint/vm/topdown/src/sync/syncer.rs","line_number":96}
"2024-02-27T19:44:01.278919Z", "message":"previous non null parent is the pending confirmation block","pending_height":1391199},"filename":"fendermint/vm/topdown/src/sync/syncer.rs","line_number":276}
"2024-02-27T19:44:01.278923Z", "message":"polling height with parent hash","height":1391200,"parent_block_hash":"9bdf3eae78d33514b3cf262cc16eca91a75845361a906606b660afadbba72390"},"filename":"fendermint/vm/topdown/src/sync/syncer.rs","line_number":156}
"2024-02-27T19:57:35.657267Z", "message":"non-null round at height, confirmed previous height","height":1391200,"confirm":1391199},"filename":"fendermint/vm/topdown/src/sync/syncer.rs","line_number":188}
"2024-02-27T19:57:38.369043Z", "message":"non-null block pushed to cache","height":1391199},"filename":"fendermint/vm/topdown/src/sync/syncer.rs","line_number":217}

In this ☝️ instance it looks like 12 minutes spent between polling the parent hash and just before fetching the data, then 3s to fetch the data and write to cache.

"2024-02-27T19:58:31.807000Z", "message":"previous non null parent is the pending confirmation block","pending_height":1391211},"filename":"fendermint/vm/topdown/src/sync/syncer.rs","line_number":276}
"2024-02-27T19:58:31.807003Z", "message":"polling height with parent hash","height":1391212,"parent_block_hash":"dd0bb9a330d94a6d2de94f496192bf4eee57930dfaac2b59cc03ea388488d3eb"},"filename":"fendermint/vm/topdown/src/sync/syncer.rs","line_number":156}
"2024-02-27T19:58:32.996770Z", "message":"non-null round at height, confirmed previous height","height":1391212,"confirm":1391211},"filename":"fendermint/vm/topdown/src/sync/syncer.rs","line_number":188}
"2024-02-27T20:15:04.075883Z", "message":"non-null block pushed to cache","height":1391211},"filename":"fendermint/vm/topdown/src/sync/syncer.rs","line_number":217}

In this ☝️ one 17 minutes go past that include fetching data and writing it to the cache.