Fix data race in BoostConnection reported by TSAN

ThreadSanitizer reported a data-race between Boost's socket
close() and async_read_some().

Test-Information:

Verified all tests still pass and that TSAN does not report
an error anymore in a scenario where a Client connects/dis-
connects randomly.

Tested on Debian 8 and OS X 10.11.3.

Change-Id: I5e705efb15bee767dd5a55539854b5e488b3bf64

Tagged swift-3.0alpha.