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.

config/danbooru_default_config.rb

@@ -299,13 +299,6 @@ module Danbooru
       []
     end
 
-    # Counting every post is typically expensive because it involves a sequential scan on
-    # potentially millions of rows. If this method returns a value, then blank searches
-    # will return that number for the fast_count call instead.
-    def blank_tag_search_fast_count
-      nil
-    end
-
     # DeviantArt login cookies. Login to DeviantArt and extract these from the browser.
     # https://github.com/danbooru/danbooru/issues/4219
     def deviantart_cookies
@@ -440,11 +433,6 @@ module Danbooru
       false
     end
 
-    # enable some (donmai-specific) optimizations for post counts
-    def estimate_post_counts
-      false
-    end
-
     # Enables recording of popular searches, missed searches, and post view
     # counts. Requires Reportbooru to be configured and running - see below.
     def enable_post_search_counts