Make column mappings name/type exclusive to a thread

We have only seen it happen once, but columns were not being
mapped and it seems it was due to threading issues with the
mapper.

also updated .gitignore to igore .ruby-*

.gitignore

@@ -5,6 +5,7 @@
 .config
 .rvmrc
 .yardoc
+.ruby-*
 Gemfile.lock
 coverage
 pkg/*

lib/protobuf/active_record/columns.rb

@@ -39,20 +39,22 @@ def _protobuf_datetime_column?(key)
         # Map out the columns for future reference on type conversion
         # :nodoc:
         def _protobuf_map_columns(force = false)
-          return unless table_exists?
+          ::Thread.exclusive do
+            @_protobuf_mapped_columns = false if force
 
-          @_protobuf_mapped_columns = false if force
-          return if _protobuf_mapped_columns?
+            return unless table_exists?
+            return if _protobuf_mapped_columns?
 
-          @_protobuf_columns = {}
-          @_protobuf_column_types = Hash.new { |h,k| h[k] = [] }
+            @_protobuf_columns = {}
+            @_protobuf_column_types = Hash.new { |h,k| h[k] = [] }
 
-          columns.map do |column|
-            @_protobuf_columns[column.name.to_sym] = column
-            @_protobuf_column_types[column.type.to_sym] << column.name.to_sym
-          end
+            columns.map do |column|
+              @_protobuf_columns[column.name.to_sym] = column
+              @_protobuf_column_types[column.type.to_sym] << column.name.to_sym
+            end
 
-          @_protobuf_mapped_columns = true
+            @_protobuf_mapped_columns = true
+          end
         end
 
         def _protobuf_mapped_columns?