Implement large csum_block_size support (more than 4k) + refactor blockstore_flush

src/allocator.h

@@ -23,3 +23,5 @@ public:
 };
 
 void bitmap_set(void *bitmap, uint64_t start, uint64_t len, uint64_t bitmap_granularity);
+void bitmap_clear(void *bitmap, uint64_t start, uint64_t len, uint64_t bitmap_granularity);
+bool bitmap_check(void *bitmap, uint64_t start, uint64_t len, uint64_t bitmap_granularity);