fix: shim setImmediate in browser

src/circuit/dialer.js

@@ -3,6 +3,7 @@
 const once = require('once')
 const PeerId = require('peer-id')
 const waterfall = require('async/waterfall')
+const setImmediate = require('async/setImmediate')
 const multiaddr = require('multiaddr')
 
 const Connection = require('interface-connection').Connection

src/circuit/hop.js

@@ -12,6 +12,7 @@ const proto = require('../protocol').CircuitRelay
 const multiaddr = require('multiaddr')
 const series = require('async/series')
 const waterfall = require('async/waterfall')
+const setImmediate = require('async/setImmediate')
 
 const multicodec = require('./../multicodec')