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/chunk.h

@@ -6,7 +6,7 @@
 #include <stdbool.h>
 #include <sys/stat.h>
 
-#define DESIRED_CHUNK_SIZE 10 * 1024 * 1024
+#define DESIRED_CHUNK_SIZE (10 * 1024 * 1024)
 
 typedef struct {
   File **items;