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/src/main.cpp b/src/main.cpp
index 5cf6c10..bb45f55 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -61,11 +61,13 @@
             return nullptr;
         }
 
-        log<level::INFO>("Loading from config with",
-                         entry("BASE_ID=%s", config.blobBaseId.c_str()),
-                         entry("FILE=%s", config.sysFilePath.c_str()),
-                         entry("MAX_SIZE=%llx", static_cast<unsigned long long>(
-                                                    config.maxSizeBytes)));
+        log<level::INFO>(
+            "Loading from config with",
+            entry("BASE_ID=%s", config.blobBaseId.c_str()),
+            entry("FILE=%s", config.sysFilePath.c_str()),
+            entry("MAX_SIZE=%llx",
+                  static_cast<unsigned long long>(
+                      config.maxSizeBytes ? *config.maxSizeBytes : 0)));
 
         auto file = std::make_unique<binstore::SysFileImpl>(config.sysFilePath,
                                                             config.offsetBytes);