http: maintain curl sessions

http: maintain curl sessions

Allow curl sessions to be kept alive (ie. not ended with
curl_easy_cleanup()) even after the request is completed, the number of
which is determined by the configuration setting http.minSessions.

Add a count for curl sessions, and update it, across slots, when
starting and ending curl sessions.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>