Don't reset StringBuffers in loops when not needed

- Moved out creating StringBuffers in loops and instead create them
  outside and just reset the buffer if it was not allocated (to avoid
  a possible malloc/free for every entry)

Other things related to set_buffer_if_not_allocated()
- Changed Valuebuffer to not call set_buffer_if_not_allocated() when
  it is created.
- Fixed geometry functions to reset string length before calling
  String::reserve().  This is because one should not access length()
  of an undefined.
- Added Item_func_conv_charset::save_in_field() as the item is using
  str_value to store cached values, which conflicts with
  Item::save_str_in_field().
- Changed Item_proc_string to not store the string value in sql_string
  as this clashes with Item::save_str_in_field().
- Locally store value of full_name_cstring() in analyse::end_of_records()
  as Item::save_str_in_field() may overwrite it.
- Marked some strings as set_thread_specific()
- Added String::free_buffer() to be used internally in String functions
  to just free the buffer but not reset other String values.
- Fixed uses_buffer_owned_by() to check for allocated length instead of
  strlength, which could be marked MEM_UNDEFINED().

sql/sql_string.cc

@@ -40,7 +40,7 @@ bool Binary_string::real_alloc(size_t length)
   str_length=0;
   if (Alloced_length < arg_length)
   {
-    free();
+    free_buffer();
     if (!(Ptr=(char*) my_malloc(STRING_PSI_MEMORY_KEY,
                                 arg_length,MYF(MY_WME | (thread_specific ?
                                                 MY_THREAD_SPECIFIC : 0)))))
@@ -1112,7 +1112,8 @@ uint
 String_copier::well_formed_copy(CHARSET_INFO *to_cs,
                                 char *to, size_t to_length,
                                 CHARSET_INFO *from_cs,
-                                const char *from, size_t from_length, size_t nchars)
+                                const char *from, size_t from_length,
+                                size_t nchars)
 {
   if ((to_cs == &my_charset_bin) || 
       (from_cs == &my_charset_bin) ||