Add missing argument max_buffer_size to TCPClient.connect in SimpleAsyncHTTPClient
Fix a subtle reference cycle that can lead to increased memory consumption.

Closes #1165.
Limit the number of connections we will accept per call to accept_handler.

This ensures that other callbacks scheduled on the IOLoop have a chance
to run.  This shows up as increased memory usage in benchmark scenarios
(since 4.0 allowed simple http transactions to be processed synchronously).
Fix an erroneous return of None in IOStream.connect.

This causes AttributeErrors in TCPClient, although I can only
reproduce this case reliably on freebsd.

Closes #1168.
Add release notes for 4.0.2.