change lz4compress etc. to lz4_compress for php compatibility

The php implementation of lz4 compress/decompression has method names lz4_compress and lz4_decompress. HPHP supplies lz4compress and lz4decompress (without spaces). The causes an issue with compatibility of code when running on PHP or HHVM.

https://github.com/kjdev/php-ext-lz4
https://github.com/colinmollenhour/credis/issues/19
https://github.com/colinmollenhour/Cm_RedisSession/issues/31

This renames those methods to lz4_compress, lz4_decompress and lz4_hccompress