http: add reusedSocket property on client request

Currently It's hard to handle keep-alive connection closes at unfortunate time. This PR set ClientRequest.reusedSocket property when reusing socket for request, so user can handle retry base on wether the request is reusing a socket. Similar to what chromium did

Refs:

• Request fail to retry: https://github.com/request/request/issues/3131
• Discussion in golang for the same issue: https://github.com/golang/go/issues/22158

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines