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/client/scanner.h

@@ -11,9 +11,10 @@ typedef struct {
   DIR *current_dir;
   char *current_path;
   bool use_metadata;
+  unsigned long long chunk_size;
 } DirectoryScanner;
 
-DirectoryScanner *directory_scanner_create(char *root_directory, bool use_metadata);
+DirectoryScanner *directory_scanner_create(char *root_directory, bool use_metadata, unsigned long long chunk_size);
 Chunk *directory_scanner_next(DirectoryScanner *scanner);
 void directory_scanner_destroy(DirectoryScanner *scanner);