perftools: add -v option to pin the (D)TLS protocol version in writeread #2059

Recent merge of the dtls-1.3 feature branch affected on writeread performance test. By default it uses the highest (D)TLS version which was DTLS 1.2 unutil recently. On the chart it looks like regression but in fact we started to test a new protocol version. In openssl/perftools#96 I introduce a new option -v for writeread test to specify the protocol version explicitly.