Fix retrying of SocketTimeoutExceptions in HttpConnector

[artem-zinnatullin]

As part of investigation of #8974 I found that recent change 982e0b8 broke retries of SocketTimeoutException in HttpConnector, intention of that change was good but lack of tests resulted in broken logic.

IntelliJ highlights the problem — branch with instanceof SocketTimeoutException would have never been executed:

---

This PR adds missing tests and fixes the logic to still present SocketTimeoutException as IOException for upstream consumers while handling it properly internally in the HttpConnector.

[aehlig]

LGTM. Thanks.

[philwo]

Hi @artem-zinnatullin,

the test you added in this PR seems to be flaky at time, for example see here:

https://buildkite.com/bazel/bazel-bazel/builds/9795#175f3020-00fd-4163-bfc6-716f7eb94662
https://buildkite.com/bazel/bazel-bazel/builds/9791#15545e80-f124-422d-9c51-09c681118506
https://buildkite.com/bazel/bazel-bazel/builds/9790#6c4ed173-9a61-4342-81a7-1e76b8268df6

You can find the logs for the failed test in the "Artifacts" tab. The log always looks like this and it only seems to happen on Windows:

exec ${PAGER:-/usr/bin/less} "$0" || exit 1
Executing tests from //src/test/java/com/google/devtools/build/lib/bazel/repository/downloader:DownloaderTestSuite
-----------------------------------------------------------------------------
JUnit4 Test Runner
..................E..............................................................................
Time: 28.809
There was 1 failure:
1) socketTimeout_retries(com.google.devtools.build.lib.bazel.repository.downloader.HttpConnectorTest)
expected: 1
but was : 2
	at com.google.devtools.build.lib.bazel.repository.downloader.HttpConnectorTest.socketTimeout_retries(HttpConnectorTest.java:323)

FAILURES!!!
Tests run: 96,  Failures: 1


BazelTestRunner exiting with a return value of 1
JVM shutdown hooks (if any) will run now.
The JVM will exit once they complete.

-- JVM shutdown starting at 2019-09-13 11:47:54 --

Do you have an idea what could cause this? I will deactivate the test temporarily while we're figuring this out.