Node-to-node TLS support

These changes add (optional) node-to-node TLS encryption & authentication using certificates.

Some TODOs left, but what's here should be OK for review.

CI running without actually *using* TLS (i.e. no configuration, using plain sockets only), base & quick passed.

TODO:

 * [x] Add `mkcerts.sh` script for CI purposes
 * [x] Add simple testcase to validate nodes use TLS when configured to, e.g. using `echo -n "" | openssl s_client -connect localhost:4010 -CAfile ca/cacert.pem -cert ca/arakoon1.pem -key ca/arakoon1.key 2>&1 | grep "Verify return code"`
 * [ ] Add more elaborate testcases for reconnecting behavior etc (re-use system tests)