ssb: Add some plausible API and a table for storing instance-wide blocks.

src/ssb.db.h

@@ -617,4 +617,18 @@ tf_ssb_identity_info_t* tf_ssb_db_get_identity_info(tf_ssb_t* ssb, const char* u
 */
 void tf_ssb_db_add_blob_wants(sqlite3* db, const char* id);
 
+/**
+** Add an instance-wide block.
+** @param db The database.
+** @param id The account, message, or blob ID to block.
+*/
+void tf_ssb_db_add_block(sqlite3* db, const char* id);
+
+/**
+** Remove an instance-wide block.
+** @param db The database.
+** @param id The account, message, or blob ID to unblock.
+*/
+void tf_ssb_db_remove_block(sqlite3* db, const char* id);
+
 /** @} */