perf: downsized test data, increased SSH socket buffer, and made chunk size configurable

- Reduced test data from 50MB to 25MB (test.py)
- Increased SSH socketpair buffer to 1MB via setsockopt (transport_ssh.c)
- Made chunk size configurable: new Config field, --chunk-size CLI flag,
  sent over wire, and used in scanner instead of DESIRED_CHUNK_SIZE
- Fixed DESIRED_CHUNK_SIZE macro parentheses (chunk.h)

src/shared/transport_ssh.c

@@ -55,6 +55,12 @@ Client *client_connect_ssh(char *destination) {
     exit(EXIT_FAILURE);
   }
 
+  int buf_size = 1024 * 1024;
+  setsockopt(sv[0], SOL_SOCKET, SO_SNDBUF, &buf_size, sizeof(buf_size));
+  setsockopt(sv[0], SOL_SOCKET, SO_RCVBUF, &buf_size, sizeof(buf_size));
+  setsockopt(sv[1], SOL_SOCKET, SO_SNDBUF, &buf_size, sizeof(buf_size));
+  setsockopt(sv[1], SOL_SOCKET, SO_RCVBUF, &buf_size, sizeof(buf_size));
+
   int exec_pipe[2];
   if (pipe(exec_pipe) < 0) {
     perror("pipe failed");