Instead of passing the easy handle directly to nghttp2 to map from
stream_id to our structs, we create an 'easymap' node (for each new
stream) and add to a per-connection linked list. The easymap struct
itself contains a pointer to the easy handle and the stream id. This
node *MUST* remain in the list until the 'on_stream_close' callback
comes. This node can survive the easy handle.

If we want to remove the association between a stream and an easy handle
before the stream end callback from nghtp2, we now call
disassociate_easymap() which "soft removes" the association in the
easymap struct only.

This is necesary since when nghttp2_submit_request() is called, the
stream<=>easy association isn't immediately done by nghttp2 and if we
then try to clear the assication at once (using
nghttp2_session_set_stream_user_data), that clearing fails and we risk
having the (by now) old assication getting done by nghttp2 and then risk
subsequently fetching a stale pointer to already freed data.

Fixes #2688
Fixes #2894

... before the stream is started, we have it set to -1.

Fixes #2894

... before the stream is started, we have it set to -1.

Fixes curl#2894
Closes curl#2898

... before the stream is started, we have it set to -1.

Fixes curl#2894
Closes curl#2898