fix memory leak in AcceptorTest
fix memory leak in AcceptorTest

Summary: caught by "--clang --sanitize address"

Test Plan:
fbconfig --clang --sanitize address 'proxygen/lib/services' && fbmake runtests

before fix:
==2560306==ERROR: LeakSanitizer: detected memory leaks
#0 0xce906c in operator new(unsigned long) /home/engshare/third-party2/llvm_compiler_rt/3.6/src/compiler-rt.git-release_36/lib/asan/asan_new_delete.cc:62
#1 0x431b82 in TestAcceptor::onNewConnection(std::unique_ptr<folly::AsyncSocket, folly::DelayedDestruction::Destructor>, folly::SocketAddress const*, std::basic_fbstring<
char, std::char_traits<char>, std::allocator<char>, std::fbstring_core<char> > const&, folly::TransportInfo const&) /tmp/proxygen/lib/services/test/AcceptorTest.cpp:45

after fix:
proxygen/lib/services:acceptor_test - AcceptorTest.Basic...ok (0s)
Test Results Summary:
Passed: 2
100% successful

Reviewed By: xiaosophiawang@fb.com

Subscribers: bmatheny, yfeldblum

FB internal diff: D2304013

Tasks: 7885747

LilMeyer/proxygen,hiproz/proxygen,chenmoshushi/proxygen,chenmoshushi/proxygen,KublaikhanGeek/proxygen,hongliangzhao/proxygen,chenmoshushi/proxygen,supriyantomaftuh/proxygen,raphaelamorim/proxygen,fqihangf/proxygen,hongliangzhao/proxygen,supriyantomaftuh/proxygen,pueril/proxygen,hiproz/proxygen,zhiweicai/proxygen,raphaelamorim/proxygen,chenmoshushi/proxygen,pueril/proxygen,pueril/proxygen,KublaikhanGeek/proxygen,LilMeyer/proxygen,jgli/proxygen,songfj/proxygen,hongliangzhao/proxygen,jgli/proxygen,LilMeyer/proxygen,supriyantomaftuh/proxygen,wrmsr/proxygen,wrmsr/proxygen,songfj/proxygen,zhiweicai/proxygen,jgli/proxygen,hnutank163/proxygen,wrmsr/proxygen,hnutank163/proxygen,Orvid/proxygen,songfj/proxygen,raphaelamorim/proxygen,supriyantomaftuh/proxygen,hiproz/proxygen,Orvid/proxygen,hnutank163/proxygen,Werror/proxygen,raphaelamorim/proxygen,songfj/proxygen,zhiweicai/proxygen,Werror/proxygen,Orvid/proxygen,fqihangf/proxygen,pueril/proxygen,Orvid/proxygen,hongliangzhao/proxygen,hiproz/proxygen,KublaikhanGeek/proxygen,hnutank163/proxygen,zhiweicai/proxygen,jgli/proxygen,Werror/proxygen,KublaikhanGeek/proxygen,Werror/proxygen,LilMeyer/proxygen,fqihangf/proxygen,wrmsr/proxygen,fqihangf/proxygen