Add miniz, a drop-in replacement for (parts of) zlib

miniz is a single-file drop-in replacement for most of zlib's
functionality. Having this bundled is quite convenient for us, so we
can rely on compression even on platforms where zlib is not readily
available.

miniz allows disabling unneeded functionality at compile time, by changing
some #defines, so we just compile the parts we really need (namely, the
basic DEFLATE routines).

We may use zlib proper in the future if we find it at compile time, though.
Implement streaming compression support

This leverages miniz in order to DEFLATE the data stream between
client and core. This is the standard format supported by zlib.

Note that streaming compression currently is unconditionally enabled
for the DataStream protocol, and unconditionally disabled for the
legacy protocol. We'll make them honor the handshake negotiation
later.
Enable the DataStream protocol by default

Now that compression works, let's use the DataStream protocol
by default if both sides claim support for it.