Make Draco the codeowner of the blockdb (#4187)

Author: StephenButtolph

.github/CODEOWNERS

@@ -28,5 +28,6 @@
 /tests/ @joshua-kim @maru-ava
 /tests/*.md @joshua-kim @maru-ava @meaghanfitzgerald
 /tests/reexecute/ @aaronbuchwald
+/x/blockdb @DracoLi
 /x/merkledb @joshua-kim @rrazvan1
 /x/sync @joshua-kim @rrazvan1