[ajax, IE] rewrite some code to avoid duplicated evaluation with IE

[repo, hooks] small task optimization: don't start cleanup_old_transactions when undo support is not activated, and retrieve a session to update a source only when some source to update is found

[http] closes #1816411, restore expires_header on static files in non-debug mode

[native source] fix bug introduced in 7645:cb2459aaa7d7: fetchone() may return None

[source] closes #1816412: try to reconnect to dbms if connection is reset in eid_type_source()

[twisted] add an option to configure twisted's threadpool size Twisted's threadpool size defaults to 10, which means we virtually try to serve up to 10 simulateneous requests but connection-pool size defaults to 4 on CW-server side. That means that under heavy load, chances are high that some HTTP queries end up with a 500 error code / connection-pool exhausted.

[book, form] include complete example of self-posted form with custom field/widget + minor other changes

backport stable

[datafeed] don't raise bad config error on source initialization

[schema] closes #1816439: RQLConstraint does not take variable U into account