Avoid a hash object allocation per each query/execute call (#1112)

This is a same optimization approach with ruby/ruby#2393.

Author: kamipo

lib/mysql2.rb

@@ -82,5 +82,6 @@ def self.key_hash_as_symbols(hash)
     else
       ::Timeout::Error
     end
+    TIMEOUT_ERROR_NEVER = { TIMEOUT_ERROR_CLASS => :never }.freeze
   end
 end

lib/mysql2/client.rb

@@ -127,7 +127,7 @@ def parse_connect_attrs(conn_attrs)
     end
 
     def query(sql, options = {})
-      Thread.handle_interrupt(::Mysql2::Util::TIMEOUT_ERROR_CLASS => :never) do
+      Thread.handle_interrupt(::Mysql2::Util::TIMEOUT_ERROR_NEVER) do
         _query(sql, @query_options.merge(options))
       end
     end

lib/mysql2/statement.rb

@@ -1,7 +1,7 @@
 module Mysql2
   class Statement
     def execute(*args, **kwargs)
-      Thread.handle_interrupt(::Mysql2::Util::TIMEOUT_ERROR_CLASS => :never) do
+      Thread.handle_interrupt(::Mysql2::Util::TIMEOUT_ERROR_NEVER) do
         _execute(*args, **kwargs)
       end
     end