feat(lsm): implement basic remove

lsm/include/lsm/store.h

@@ -176,6 +176,13 @@ void lsm_entry_close(lsm_entry_handle *handle);
 lsm_error lsm_store_insert(lsm_entry_handle **out, lsm_store *store,
                            lsm_str *key);
 
+/**
+ * Mark the entry as removed.
+ *
+ * @param handle handle to entry to remove
+ */
+void lsm_entry_remove(lsm_entry_handle *handle);
+
 /**
  * Append new data to the given entry, which is expected to be in the store.
  *