Add igbinary to replace PHP serialization and deserialization

[andrewholgate]

igbinary has been shown to have improved performance and size over standard PHP serializsation and deserializsation.

When the PHP module is working for PHP7 (igbinary/igbinary7#1), install it and perform basic benchmarking of performance against PHP's native serialize() and json_decode, etc.

• phpredis can use igbinary by setting the flag: ./configure --enable-redis-igbinary
• For improved caching performance, igbinary cache could be ported to D8.