fixing context init for multi threads

[hcheng2002cn]

unit test with the script:

from pglast import parse_sql
import time
import threading

#cmd = 'SELECT * FROM (SELECT DISTINCT ON (Item) "Item", "Location" FROM "adam/UN"."t10" ORDER BY "Item", "Location" DESC) ORDER BY "Location" DESC;'
cmd = 'SELECT * FROM (SELECT DISTINCT ON (Item) "Item", "Location" FROM "adam/UN"."t10" ORDER BY "Item", "Location" DESC) a ORDER BY "Location" DESC;'

def parser():
	try:
		try:
			parse_sql(cmd)
            print("all good.")
		except Exception as e:
			print(e)
			raise
	except:
		time.sleep(1)
		pass

for i in range(100):
	x = threading.Thread(target=parser)
	time.sleep(1)
	x.start()

while 1:
	pass