[patch]thread safety connection pool for all backend

delete AutoReleaseCursor? , it breaks transaction in multithread when 'threadsafety > 1'.

update the connection pool in magic-removal branch. only the next two diffs needed.

pool.py.2:

• fix check_thread close not thread safety connetion. now thread safety < 2 connection will not run check thread. so you must use zhe connection in the web server which has thread pool.
• fix cannot stop the check_thread in manage.py shell. use atexit.register function for this problem.

django.db.__init__.py.diff:

• add new setting USE_DBPOOL.

and now ,you must implement get_check_sql function in django.db.bachend.xxxx.base.

sqlite3:

def get_check_sql():
    return "SELECT name FROM sqlite_master WHERE type='table' limit 1"

ado_mssql:

def get_check_sql():
    return "select top 1 1 from sysfiles"

postgresql:

def get_check_sql():
    return "SELECT c.relname FROM pg_catalog.pg_class limit 1"

Superseded by #1442

Milestone Version 1.0 deleted