Remove debug logging spam when syncing runtime's rounds

SUMMARY

Remove debug logging spam when syncing runtime's rounds.

Currently, a debug message is emitted for each round that is being prepared for syncing:

... trimmed ...

{"awaiting_retry":"outstanding_mask{state-root}","caller":"node.go:1255","level":"debug","module":"worker/storage/committee","msg":"preparing round sync","outstanding_mask":"outstanding_mask{}","round":20134,"runtime_id":"00000000000000000000000000000000000000000000000072c8215e60d5bca7","ts":"2021-12-16T10:52:57.539461527Z"}
{"awaiting_retry":"outstanding_mask{state-root}","caller":"node.go:1255","level":"debug","module":"worker/storage/committee","msg":"preparing round sync","outstanding_mask":"outstanding_mask{}","round":20135,"runtime_id":"00000000000000000000000000000000000000000000000072c8215e60d5bca7","ts":"2021-12-16T10:52:57.539484704Z"}
{"awaiting_retry":"outstanding_mask{state-root}","caller":"node.go:1255","level":"debug","module":"worker/storage/committee","msg":"preparing round sync","outstanding_mask":"outstanding_mask{}","round":20136,"runtime_id":"00000000000000000000000000000000000000000000000072c8215e60d5bca7","ts":"2021-12-16T10:52:57.539514816Z"}
{"awaiting_retry":"outstanding_mask{state-root}","caller":"node.go:1255","level":"debug","module":"worker/storage/committee","msg":"preparing round sync","outstanding_mask":"outstanding_mask{}","round":20137,"runtime_id":"00000000000000000000000000000000000000000000000072c8215e60d5bca7","ts":"2021-12-16T10:52:57.539533801Z"}
{"awaiting_retry":"outstanding_mask{state-root}","caller":"node.go:1255","level":"debug","module":"worker/storage/committee","msg":"preparing round sync","outstanding_mask":"outstanding_mask{}","round":20138,"runtime_id":"00000000000000000000000000000000000000000000000072c8215e60d5bca7","ts":"2021-12-16T10:52:57.539547648Z"}
{"awaiting_retry":"outstanding_mask{state-root}","caller":"node.go:1255","level":"debug","module":"worker/storage/committee","msg":"preparing round sync","outstanding_mask":"outstanding_mask{}","round":20139,"runtime_id":"00000000000000000000000000000000000000000000000072c8215e60d5bca7","ts":"2021-12-16T10:52:57.539561767Z"}

... trimmed ...

For example, in case there are 100k rounds to be synced, it would emit 100k debug log messages at once.

ISSUE TYPE

• Feature Idea

Start in the round-sync path around node.go:1255, identified by the log's caller field, and inspect where the per-round "preparing round sync" message is emitted. Done means syncing many rounds no longer produces one debug log message for each round; verify that the synchronization behavior remains intact.