Lower the tablesample thresholds/targets.

The numbers in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22650#note_117645395
are quite promising with that target.

lib/gitlab/database/count/tablesample_count_strategy.rb

@@ -14,8 +14,8 @@ module Gitlab
       #
       # There are no guarantees with respect to the accuracy of the result or runtime.
       class TablesampleCountStrategy < ReltuplesCountStrategy
-        EXACT_COUNT_THRESHOLD = 100_000
-        TABLESAMPLE_ROW_TARGET = 100_000
+        EXACT_COUNT_THRESHOLD = 10_000
+        TABLESAMPLE_ROW_TARGET = 10_000
 
         def count
           estimates = size_estimates(check_statistics: false)