Performance improvements made.
ExtractValue for large XML values is now much faster
(about 2000 times faster of 1Mb-long XML values).


sql/item_xmlfunc.cc:
  Performance improvement for huge XML values:
  1. Avoid reallocs - reserve extra memory:
  using String::reserve() + String::q_append()
  instead of String::append()
  2. Parent is now not searched through the all previous
  nodes in backward direction. Instead, we do the following:
  - we introduce data->parent - a new member in MY_XML_USER_DATA
  - when entering a new tag/attribute node, we remember offset
    of the current node in data->parent
  - when leaving a tag/attribute node, we change data->parent
    to offset of the parent of the current node.