Fix exit table, set delayed_withdrawn in exits

- In exit table, `instant_withdrawn`, `delayed_withdraw_request`, and
  `delayed_withdrawn` were referencing batch_num.  But these actions happen
  outside a batch, so they should reference a block_num.
- Process delayed withdrawns:
    - In Synchronizer, first match a Rollup delayed withdrawn request, with the
      WDelayer deposit (via TxHash), and store the owner and token associated
      with the delayed withdrawn.
    - In HistoryDB: store the owner and token of a delayed withdrawal request
      in the exit_tree, and set delayed_withdrawn when the withdraw is done in
      the WDelayer.
- Update dependency of sqlx to master
    - Last release of sqlx is from 2018 October, and it doesn't support
      `NamedQuery` with a slice of structs, which is used in this commit.

db/migrations/0001.sql

@@ -109,9 +109,11 @@ CREATE TABLE exit_tree (
     account_idx BIGINT REFERENCES account (idx) ON DELETE CASCADE,
     merkle_proof BYTEA NOT NULL,
     balance BYTEA NOT NULL,
-    instant_withdrawn BIGINT REFERENCES batch (batch_num) ON DELETE SET NULL,
-    delayed_withdraw_request BIGINT REFERENCES batch (batch_num) ON DELETE SET NULL,
-    delayed_withdrawn BIGINT REFERENCES batch (batch_num) ON DELETE SET NULL
+    instant_withdrawn BIGINT REFERENCES block (eth_block_num) ON DELETE SET NULL,
+    delayed_withdraw_request BIGINT REFERENCES block (eth_block_num) ON DELETE SET NULL,
+    owner BYTEA,
+    token BYTEA,
+    delayed_withdrawn BIGINT REFERENCES block (eth_block_num) ON DELETE SET NULL
 );
 
 -- +migrate StatementBegin