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.h

@@ -1,28 +0,0 @@
-#ifndef COMPRESSOR_H
-#define COMPRESSOR_H
-
-#include <string>
-
-namespace Network {
-  class Compressor {
-  private:
-    static const int BUFFER_SIZE = 2048 * 2048; /* effective limit on terminal size */
-
-    unsigned char *buffer;
-
-  public:
-    Compressor() : buffer( NULL ) { buffer = new unsigned char[ BUFFER_SIZE ]; }
-    ~Compressor() { if ( buffer ) { delete[] buffer; } }
-
-    std::string compress_str( const std::string &input );
-    std::string uncompress_str( const std::string &input );
-
-    /* unused */
-    Compressor( const Compressor & );
-    Compressor & operator=( const Compressor & );
-  };
-
-  Compressor & get_compressor( void );
-}
-
-#endif