SOLR-18348: Move building suggester to async process outside of commit lifecycle.

[epugh]

https://issues.apache.org/jira/browse/SOLR-18348

Description

See the original JIRA for the problem. Basically, commits take longer when you have the suggester get built.

Solution

Messing around so far...

Tests

Please describe the tests you've developed or run to confirm this patch implements the feature or solves the problem.

Checklist

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[epugh]

Two scenarios, since the response shape differs — full detail on a single replica, just the yes/no flag on a distributed query.

Direct query to one replica (distrib=false, or a standalone core) — full detail, per suggester:

GET /solr/mycollection_shard1_replica_n1/suggest?suggest=true&suggest.dictionary=slowSuggester&suggest.q=elec&suggest.count=5&distrib=false&wt=json

{
  "responseHeader": { "status": 0, "QTime": 3 },
  "suggest": {
    "slowSuggester": {
      "elec": {
        "numFound": 2,
        "suggestions": [
          { "term": "electronics", "weight": 100, "payload": "" },
          { "term": "electric fan", "weight": 80, "payload": "" }
        ]
      }
    }
  },
  "suggesterIndexVersions": {
    "slowSuggester": {
      "builtFromIndexVersion": 41,
      "currentIndexVersion": 43,
      "stale": true
    }
  }
}

Normal distributed SolrCloud query (fans out to all shards, gets merged) — just the aggregate flag, no per-shard numbers:

GET /solr/mycollection/suggest?suggest=true&suggest.dictionary=slowSuggester&suggest.q=elec&suggest.count=5&wt=json

{
  "responseHeader": { "status": 0, "QTime": 12 },
  "suggest": {
    "slowSuggester": {
      "elec": {
        "numFound": 2,
        "suggestions": [
          { "term": "electronics", "weight": 100, "payload": "" },
          { "term": "electric fan", "weight": 80, "payload": "" }
        ]
      }
    }
  },
  "suggesterStale": true
}

suggesterStale: true just means at least one of the shards that answered this query was serving from a suggester that hadn't caught up yet — it doesn't say which shard or by how much. To pin that down you'd fall back to the first example (hit a specific replica directly with distrib=false) or check /admin/metrics.