[ConnectionPool] Fix race between shutdown and backoff timer

[fabianfett]

Apparently I introduced a flaky test that showed up here:
https://ci.swiftserver.group/job/async-http-client-swift53-prb/403/console

Test Case 'HTTPConnectionPoolTests.testConnectionCreationIsRetriedUntilPoolIsShutdown' started at 2021-09-09 13:48:55.778 Assertion failed: At least this connection is currently in backoff: file AsyncHTTPClient/HTTPConnectionPool+HTTP1StateMachine.swift, line 171 Current stack trace: 0 libswiftCore.so 0x00007f99eae1d990 swift_reportError + 50 1 libswiftCore.so 0x00007f99eae91260 _swift_stdlib_reportFatalErrorInFile + 115 2 libswiftCore.so 0x00007f99eab76925 <unavailable> + 1399077 3 libswiftCore.so 0x00007f99eab76567 <unavailable> + 1398119 4 libswiftCore.so 0x00007f99eab76b02 <unavailable> + 1399554 5 libswiftCore.so 0x00007f99eab74fa0 _assertionFailure(_:_:file:line:flags:) + 517 6 async-http-clientPackageTests.xctest 0x0000562a0c50e4b3 <unavailable> + 1422515 7 async-http-clientPackageTests.xctest 0x0000562a0c518156 <unavailable> + 1462614 8 async-http-clientPackageTests.xctest 0x0000562a0c4deac4 <unavailable> + 1227460 9 async-http-clientPackageTests.xctest 0x0000562a0c4e5b61 <unavailable> + 1256289 10 async-http-clientPackageTests.xctest 0x0000562a0c4da7aa <unavailable> + 1210282 11 async-http-clientPackageTests.xctest 0x0000562a0c4e5b84 <unavailable> + 1256324 12 async-http-clientPackageTests.xctest 0x0000562a0caa9b24 <unavailable> + 7301924 13 async-http-clientPackageTests.xctest 0x0000562a0c4de80b <unavailable> + 1226763 14 async-http-clientPackageTests.xctest 0x0000562a0c48ec1f <unavailable> + 900127 15 async-http-clientPackageTests.xctest 0x0000562a0c4e5ac4 <unavailable> + 1256132 16 async-http-clientPackageTests.xctest 0x0000562a0ce3e759 <unavailable> + 11056985 17 async-http-clientPackageTests.xctest 0x0000562a0cbf263c <unavailable> + 8648252 18 async-http-clientPackageTests.xctest 0x0000562a0ce46921 <unavailable> + 11090209 19 async-http-clientPackageTests.xctest 0x0000562a0cbefc4c <unavailable> + 8637516 20 async-http-clientPackageTests.xctest 0x0000562a0ce41ef7 <unavailable> + 11071223 21 async-http-clientPackageTests.xctest 0x0000562a0c4de0df <unavailable> + 1224927 22 async-http-clientPackageTests.xctest 0x0000562a0ce45164 <unavailable> + 11084132 23 async-http-clientPackageTests.xctest 0x0000562a0ce3b052 <unavailable> + 11042898 24 async-http-clientPackageTests.xctest 0x0000562a0ce401cc <unavailable> + 11063756 25 async-http-clientPackageTests.xctest 0x0000562a0ce0251b <unavailable> + 10810651 26 async-http-clientPackageTests.xctest 0x0000562a0ce02cdf <unavailable> + 10812639 27 async-http-clientPackageTests.xctest 0x0000562a0ce088e3 <unavailable> + 10836195 28 async-http-clientPackageTests.xctest 0x0000562a0ce00fdf <unavailable> + 10805215 29 async-http-clientPackageTests.xctest 0x0000562a0ce86e51 <unavailable> + 11353681 30 async-http-clientPackageTests.xctest 0x0000562a0ce8a3de <unavailable> + 11367390 31 async-http-clientPackageTests.xctest 0x0000562a0ce8a589 <unavailable> + 11367817 32 libpthread.so.0 0x00007f99ea8096db <unavailable> + 30427 33 libc.so.6 0x00007f99e89926e0 clone + 63 Exited with signal code 4 

This pr fixes this assertion.