binarystore: Add std::optional for optional config options

The `maxSizeBytes` was using 0 size by default instead of skipping it.
Letting `config.maxSizeBytes` be std::optional allow us to have more
control to see if the 0 value is expected or nullopt.

Change-Id: I128216825609909c489cbd5471b324d88d3aa25b
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/include/sys_file_impl.hpp b/include/sys_file_impl.hpp
index c74c965..fa6f5ec 100644
--- a/include/sys_file_impl.hpp
+++ b/include/sys_file_impl.hpp
@@ -6,6 +6,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 
+#include <optional>
 #include <string>
 
 namespace binstore
@@ -21,7 +22,8 @@
      *     actually reads underlying file at 'offset'
      * @param sys Syscall operation interface
      */
-    explicit SysFileImpl(const std::string& path, size_t offset = 0,
+    explicit SysFileImpl(const std::string& path,
+                         std::optional<size_t> offset = std::nullopt,
                          const internal::Sys* sys = &internal::sys_impl);
     ~SysFileImpl();
     SysFileImpl() = delete;