Summary:
We do a lot of copying of TransportInfo in proxygen, and in most cases
the SSL structs don't change after connection establishment. We could
cut down on memory usage by sharing these huge strings. This is
especially true with SPDY where all streams belonging to a session could
share these fields.

Facebook:

Test Plan: Unit tests, will canary

Reviewed By: afrind@fb.com

Subscribers: fugalh, bmatheny, ssl-diffs@, folly-diffs@, jsedgwick, woo

FB internal diff: D1807557

Tasks: 5343753

Signature: t1:1807557:1422472932:53038345fca620632097586fb9e410bca8fe748d