zlib: use more memory for a small deflate speedup.

The LIT_MEM option uses slightly more memory (for base gzip(1),
about 16kiB; according to the author, about 6% for default deflate
settings) for a small speedup.

The performance gain is more noticeable for input data with higher
entropy and less significant for data that is highly compressible,
such as source code and logs.

MFC after: 1 month