close idle HTTPDownstreamSessions before load shedding

close idle HTTPDownstreamSessions before load shedding

Summary:
when any system resource limit is reached, proxygen reduces the number of idle downstream sessions to accomodate new ones.

Test Plan:
canarying on edge241.01.ams3.  Here is the number of idle connection closed during pre load shedding stage.

https://www.facebook.com/pxlcld/ml7J

Reviewed By: afrind@fb.com

Subscribers: alandau, noamler, fugalh, bmatheny, folly-diffs@, jsedgwick, yfeldblum, chalfant, xning, alexkr

FB internal diff: D2030988

Tasks: 5698711

Signature: t1:2030988:1431369559:ce7328d51c7fd0afa7e9e5c19b0c66736d01fee1