search: refactor fast_count to return nil on timeout.

* Refactor fast_count to return nil instead of 1,000,000 if the exact count times out.
* Remove the estimate_post_counts and blank_tag_search_fast_count global config options.
* Replace the hardcoded post count estimates inside fast_count with a
  method that parses Postgres's estimated row count from EXPLAIN.

* /counts/posts.json:
** Remove the `raise_on_timeout` parameter.
** Add an `estimate_count=<true|false>` parameter.
** Return null instead of 1,000,000 if the exact count times out.

app/logical/explain_parser.rb

@@ -0,0 +1,15 @@
+class ExplainParser < Struct.new(:sql)
+  extend Memoist
+
+  def query_plan
+    result = ApplicationRecord.connection.select_one("EXPLAIN (FORMAT JSON) #{sql}")
+    json = JSON.parse(result["QUERY PLAN"])
+    json.first["Plan"]
+  end
+
+  def row_count
+    query_plan["Plan Rows"]
+  end
+
+  memoize :query_plan
+end