disconnect: separate connections and easy handles better

disconnect: separate connections and easy handles better

Do not assume/store assocation between a given easy handle and the
connection if it can be avoided.

Long-term, the 'conn->data' pointer should probably be removed as it is a
little too error-prone. Still used very widely though.

Backported patch details:
https://github.com/curl/curl/pull/3400/commits/fb445a1e18d12f577964c9347bc5bca74b37cd08

Change-Id: I18aa2cb7097b8598c90ddf8c8c68a9fecd86e295
Signed-off-by: Niraj Kumar Goit <niraj.g@samsung.com>