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)

test.py

@@ -123,7 +123,7 @@ def generate_test_files(source_dir):
         shutil.rmtree(source_dir)
     os.makedirs(source_dir)
 
-    target_total = 50 * 1024 * 1024
+    target_total = 25 * 1024 * 1024
     written = 0
 
     files = {