Use protobuf’s Gzip{Input,Output}Stream wrapper around zlib

This removes our direct zlib dependency (although of course protobuf
still uses it internally), removes a fixed 4 MiB buffer and its
corresponding limit on the terminal size, reduces some string copying,
and deletes some code.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

(Closes #230.)

src/network/compressor.cc

@@ -1,32 +0,0 @@
-#include <zlib.h>
-
-#include "compressor.h"
-#include "dos_assert.h"
-
-using namespace Network;
-using namespace std;
-
-string Compressor::compress_str( const string &input )
-{
-  long unsigned int len = BUFFER_SIZE;
-  dos_assert( Z_OK == compress( buffer, &len,
-				reinterpret_cast<const unsigned char *>( input.data() ),
-				input.size() ) );
-  return string( reinterpret_cast<char *>( buffer ), len );
-}
-
-string Compressor::uncompress_str( const string &input )
-{
-  long unsigned int len = BUFFER_SIZE;
-  dos_assert( Z_OK == uncompress( buffer, &len,
-				  reinterpret_cast<const unsigned char *>( input.data() ),
-				  input.size() ) );
-  return string( reinterpret_cast<char *>( buffer ), len );
-}
-
-/* construct on first use */
-Compressor & Network::get_compressor( void )
-{
-  static Compressor the_compressor;
-  return the_compressor;
-}