Hash-collision denial-of-service vulnerabilities (#87)

* Add case insensitive SipHash implementation
* Replace ref hash function with SipHash
* Add label to link_ref struct.
* Update find_link_ref to compare link labels as well as hashes
* Update add_link_ref to disallow duplicate entries.
* cast to char from uint8_t for strncasecmp
* update README markdown, remove TODO
* add py2 wheel generation
* fix: add logic for older glibc not having getrandom, impacting staging

Co-authored-by: Nicolaas <nweidema@usc.edu>