Delete pending txs by external mark, store tx IP

- In tx_pool, add a column called `external_delete` that can be set to true
  externally.  Regularly, the coordinator will delete all pending txs with this
  column set to true.  The interval for this action is set via the new config
  parameter `Coordinator.PurgeByExtDelInterval`.
- In tx_pool, add a column for the client ip that sent the transaction.  The
  api fills this value using the ClientIP method from gin.Context, which should
  work even under a reverse-proxy.

db/migrations/0001.sql

@@ -627,7 +627,9 @@ CREATE TABLE tx_pool (
     rq_amount BYTEA,
     rq_fee SMALLINT,
     rq_nonce BIGINT,
-    tx_type VARCHAR(40) NOT NULL
+    tx_type VARCHAR(40) NOT NULL,
+    client_ip VARCHAR,
+    external_delete BOOLEAN NOT NULL DEFAULT false
 );
 
 -- +migrate StatementBegin