Implement commit

The commit operation will serialize the binarystore protobuf and write
it to the designated sysfile location, with its size stored followed by
actual data.

Signed-off-by: Kun Yi <kunyi@google.com>
Change-Id: Idc16f410d3a1585daaddda58a3665d92a898f5c7
diff --git a/handler.cpp b/handler.cpp
index e0a343a..fbfaf47 100644
--- a/handler.cpp
+++ b/handler.cpp
@@ -151,8 +151,13 @@
 bool BinaryStoreBlobHandler::commit(uint16_t session,
                                     const std::vector<uint8_t>& data)
 {
-    // TODO: implement
-    return false;
+    auto it = sessions_.find(session);
+    if (it == sessions_.end())
+    {
+        return false;
+    }
+
+    return it->second->commit();
 }
 
 bool BinaryStoreBlobHandler::close(uint16_t session)