Fix declaration of ‘index’ shadows a global declaration [-Werror=shadow]

I have this error with some old gcc (4.6) release (Ubuntu 12.04 or Travis...)
Merge branch 'Wshadow' of https://github.com/alagoutte/nghttp2 into alagoutte-Wshadow
deflatehd: Fix crash with -t option
h2load: Return Stats rather than Worker from std::future
h2load: Use 1 thread if the number of concurrent client is 1
nghttpx: Disable SSL_CTX_set_ecdh_auto() for now
Fix off-by-one error when computing padding
nghttpx: Use std::unique_ptr<char[]> instead of char*
nghttpx: Use nullptr instead of 0
nghttpx: Use std::vector for tls_proto_list and npn_list

Now SSL/TLS option mask to disable particular SSL/TLS protocol
versions are pre-calculated and stored in Config.
nghttpx: Code cleanup
nghttpx: Use nullptr instead of NULL
nghttpx: Code cleanup
Fix warning found by -Wshorten-64-to-32
Add -Wshorten-64-to-32 to configure.ac