Move TAsyncSignalHandler into folly
Move TAsyncSignalHandler into folly

Summary:
TODO item, trying to remove unecessary dependencies on thrift

Test Plan: Unit tests

Reviewed By: davejwatson@fb.com

Subscribers: doug, fbcode-common-diffs@, davejwatson, andrewcox, alandau, bmatheny, anca, darshan, mshneer, folly-diffs@, bil, yfeldblum, haijunz, chalfant

FB internal diff: D1960215

Signature: t1:1960215:1427920934:8abd7e94c50676b05bf7ff79800df0db1bd04266

Signed-off-by: Ori Bernstein <f1fed15cf5b8387c8aee8bc535d6c2b48da91000@fb.com>

sonoble/fboss,neuhausler/fboss,raphaelamorim/fboss,peterlei/fboss,neuhausler/fboss,raphaelamorim/fboss,peterlei/fboss,sonoble/fboss,peterlei/fboss,neuhausler/fboss,biddyweb/fboss,biddyweb/fboss,peterlei/fboss,biddyweb/fboss,sonoble/fboss,raphaelamorim/fboss,neuhausler/fboss