search: cache timed out search counts.

When a search is performed, we cache the post count so we don't have to
calculate it again every time the user switches pages. However, if the
count times out, we didn't cache it before, causing us to do a slow
count on every page load. This usually happens on multi-tag searches
that return a lot of results, `1girl solo` for example.

This changes it so that the count is cached even when it times out. This
will speed up large multi-tag searches.

This also changes it so that the count is cached for a fixed 5 minutes.
Before it was variable based on the size of the count, but this probably
didn't make much difference.

app/logical/post_query_builder.rb

@@ -902,8 +902,8 @@ class PostQueryBuilder
     def fast_count(timeout: 1_000, estimate_count: true, skip_cache: false)
       count = nil
       count = estimated_count if estimate_count
-      count = cached_count if count.nil? && !skip_cache
-      count = exact_count(timeout) if count.nil?
+      count = cached_count(timeout) if count.nil? && !skip_cache
+      count = exact_count(timeout) if count.nil? && skip_cache
       count
     end
 
@@ -936,22 +936,16 @@ class PostQueryBuilder
       ExplainParser.new(build).row_count
     end
 
-    def cached_count
-      Cache.get(count_cache_key)
+    def cached_count(timeout, duration: 5.minutes)
+      Cache.get(count_cache_key, duration) do
+        exact_count(timeout)
+      end
     end
 
     def exact_count(timeout)
-      count = Post.with_timeout(timeout, nil) do
+      Post.with_timeout(timeout) do
         build.count
       end
-
-      set_cached_count(count) if count.present?
-      count
-    end
-
-    def set_cached_count(count)
-      expiry = count.seconds.clamp(3.minutes, 20.hours).to_i
-      Cache.put(count_cache_key, count, expiry)
     end
 
     def count_cache_key