[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.