Support lz4 compression library (through php-ext-lz4, https://github.com/kjdev/php-ext-lz4)

lz4 is tested to be faster than las and snappy, used this mod in production for several days before getting a new server with 64GB of RAM :-)
The only catch in this PR is that the setting must be l4z instead of lz4, since I didn't want to mess with the code too much (since you just use the first 2 digits for algo distinction, we wouldn't be able to distinct between lzf and lz4 otherwise)