threading fixed :)

Hi,

there has been a long-standing issue in the threading support in lxml,
combined with the per-thread string hash table we use for libxml2.

Here is a simple example of a sure crasher:

-------------------------------
import threading
import lxml.etree as et

xml = "<root><threadtag/></root>"

main_root = et.XML("<root/>")

def run_thread():
    thread_root = et.XML(xml)
    main_root.append(thread_root[0])
    del thread_root # deletes the document

thread = threading.Thread(target=run_thread)

thread.start()
thread.join()

print et.tostring(main_root)
-------------------------------

This crashes, because the thread parses the XML fragment into its own
dictionary and stores the tag name "threadtag" there. Then it appends the