Fix map reduces counter

Since wrap reload may execute functions multiple times, multiple map
reduces may be planned and executed.

Follows up #224

Author: DifferentialOrange

crud/select/compat/select.lua

@@ -115,7 +115,10 @@ local function build_select_iterator(space_name, user_conditions, opts)
         end
     else
         local context_stats = fiber_context.init_section('router_stats')
-        context_stats.map_reduces = 1
+        if context_stats.map_reduces == nil then
+            context_stats.map_reduces = 0
+        end
+        context_stats.map_reduces = context_stats.map_reduces + 1
     end
 
     local tuples_limit = opts.first

crud/select/compat/select_old.lua

@@ -150,7 +150,10 @@ local function build_select_iterator(space_name, user_conditions, opts)
         end
     else
         local context_stats = fiber_context.init_section('router_stats')
-        context_stats.map_reduces = 1
+        if context_stats.map_reduces == nil then
+            context_stats.map_reduces = 0
+        end
+        context_stats.map_reduces = context_stats.map_reduces + 1
     end
 
     -- generate tuples comparator