HTTP header fields stored per connection causes problems

[bagder]

The struct dynamically_allocated_data is stored within the connectdata struct. That data is then used per connection, while all the data in that struct is actually per request. A mistake.

This causes (occasional) problems for multiplexed requests as then the same connection is used for multiple (independent) requests that could have a different setup of that data.

Solution: move that data to the per-request struct held in the Curl_easy struct.