(wangle) deprecate Later and ThreadGate

(wangle) deprecate Later and ThreadGate

Summary:
Later is now superfluous. Just hold a deactivated future (e.g. one returned by `Future::via`), and then activate (or destruct) it when you're ready.
ThreadGate has proven to be too rigid and too heavyweight to be really useful. And is superfluous now with `Future::via`. e.g.
f.via(east).then(a1).then(a2).via(west).then(b1).then(b2);

This is stage one of removing these: mark them deprecated.
Stage two will be for me to change all existing usages in our code to not use them.
Stage three will be to rip these out altogether.

Test Plan:
Everything still builds and works, but now with deprecation warnings.

fbconfig -r \
common/smc/cpp/tests \
folly/wangle \
langtech/audiens \
messaging/avscan \
messaging/maelstrom \
neteng/traffic_manager \
notifications/nudge/service \
tao/client \
zeus/datashuttle
fbmake runtests

Reviewed By: jsedgwick@fb.com

Subscribers: net-systems@, fugalh, exa, njormrod, folly-diffs@

FB internal diff: D1626412

Tasks: 5409538