Summary: promote the sockets vector to a shared_ptr, since both ServerWorkerPool and ServerBootstrap use it.  Otherwise there are destruction order issues between ServerBootstrap and any IOThreadPoolExecutor you use

Test Plan: Saw use after free in D1942242, gone after this.

Reviewed By: yfeldblum@fb.com

Subscribers: chalfant, doug, fugalh, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1947553

Signature: t1:1947553:1427484417:5b78f5c9c70d244d3f52a6f71b6d1fab7b29d106