indexserver: reduce defaults for compound shards (#850)

zoekt-merge-index can OOM on instances with <=4GB mem. Hence we adjust
the defaults. Larger instances can set the ENVs to higher values.

Test plan:
N/A

cmd/zoekt-sourcegraph-indexserver/main.go

@@ -1264,8 +1264,8 @@ func (rc *rootConfig) registerRootFlags(fs *flag.FlagSet) {
 	fs.BoolVar(&rc.disableShardMerging, "shard_merging", getEnvWithDefaultBool("SRC_DISABLE_SHARD_MERGING", false), "disable shard merging")
 	fs.DurationVar(&rc.vacuumInterval, "vacuum_interval", getEnvWithDefaultDuration("SRC_VACUUM_INTERVAL", 24*time.Hour), "run vacuum this often")
 	fs.DurationVar(&rc.mergeInterval, "merge_interval", getEnvWithDefaultDuration("SRC_MERGE_INTERVAL", 8*time.Hour), "run merge this often")
-	fs.Int64Var(&rc.targetSize, "merge_target_size", getEnvWithDefaultInt64("SRC_MERGE_TARGET_SIZE", 2000), "the target size of compound shards in MiB")
-	fs.Int64Var(&rc.minSize, "merge_min_size", getEnvWithDefaultInt64("SRC_MERGE_MIN_SIZE", 1800), "the minimum size of a compound shard in MiB")
+	fs.Int64Var(&rc.targetSize, "merge_target_size", getEnvWithDefaultInt64("SRC_MERGE_TARGET_SIZE", 1000), "the target size of compound shards in MiB")
+	fs.Int64Var(&rc.minSize, "merge_min_size", getEnvWithDefaultInt64("SRC_MERGE_MIN_SIZE", 800), "the minimum size of a compound shard in MiB")
 	fs.IntVar(&rc.minAgeDays, "merge_min_age", getEnvWithDefaultInt("SRC_MERGE_MIN_AGE", 7), "the time since the last commit in days. Shards with newer commits are excluded from merging.")
 }