TLS session reuse support

[defanator]

Originally based on #183 with the following changes:

1. no separate option to disable keepalive at wrk side (this should be done either with -H "Connection: close" or at server's side (e.g. keepalive_requests 1 in nginx);
2. modern openssl versions supported;
3. no extra callbacks when TLS reuse is disabled.

[Nithya-2018]

Hi, after making changes in the wrk.c files, how did you compile them? I keep getting lua.h not found error even after installing it using sudo apt cmd.