Add fastlz decompression support to memcached extension

As of version 2.0.0b1 (current is 2.2.0), the PHP memcached extension uses
fastlz compression by default for values exceeding a certain size threshold.
The same version also added a raw length field to all compressed values
(including zlib-compressed values). The flags MEMC_VAL_COMPRESSION_FASTLZ and
MEMC_VAL_COMPRESSION_ZLIB are used to indicate the compression algorithm for
this new implementation.

This commit adds support for decompressing values that have been compressed
using fastlz or zlib using the new convention. It does not yet include support
for compressing values. Decompression support is useful because it allows PHP
and HHVM to transparently share the same memcached infrastructure.

Depends on commit hhvm/hhvm-third-party#8