fix(indexer-alt): overly chatty loud watermark updates

## Description

The loud watermark update logic always bumped the previous update
watermark by the same interval. When the indexer was running well ahead
of the loud watermark update rate, this would cause many updates to be
issued successively.

This change makes it so that the next update is always
`LOUD_WATERMARK_UPDATE_INTERVAL` away from the last loud update.

## Test plan

Run the indexer. Previously updates -- particularly from summary tables
that gathered changes up and write them out in big batches -- would come
in bursts, and this behaviour is no longer apparent after the change:

```
sui$ cargo run -p sui-indexer --release --                                       \
  --database-url "postgres://postgres:postgrespw@localhost:5432/sui_indexer_alt" \
  indexer --remote-store-url https://checkpoints.mainnet.sui.io                  \
  --last-checkpoint 1000000 --consistent-range 3600
```

crates/sui-indexer-alt/src/pipeline/concurrent/commit_watermark.rs

@@ -226,7 +226,8 @@ pub(super) fn commit_watermark<H: Handler + 'static>(
                                 }
 
                                 if watermark.checkpoint_hi_inclusive > next_loud_watermark_update {
-                                    next_loud_watermark_update += LOUD_WATERMARK_UPDATE_INTERVAL;
+                                    next_loud_watermark_update = watermark.checkpoint_hi_inclusive + LOUD_WATERMARK_UPDATE_INTERVAL;
+
                                     info!(
                                         pipeline = H::NAME,
                                         epoch = watermark.epoch_hi_inclusive,

crates/sui-indexer-alt/src/pipeline/sequential/committer.rs

@@ -316,7 +316,8 @@ pub(super) fn committer<H: Handler + 'static>(
                         .set(watermark.timestamp_ms_hi_inclusive);
 
                     if watermark.checkpoint_hi_inclusive > next_loud_watermark_update {
-                        next_loud_watermark_update += LOUD_WATERMARK_UPDATE_INTERVAL;
+                        next_loud_watermark_update = watermark.checkpoint_hi_inclusive + LOUD_WATERMARK_UPDATE_INTERVAL;
+
                         info!(
                             pipeline = H::NAME,
                             epoch = watermark.epoch_hi_inclusive,