Do not add bad connections back to pool

See knex#452.

This is a hacky workaround proof-of-concept conversation starter that
fixes the issue, but should be done in a cross-dialect way, and it
should also include tests.

lib/pool.js

@@ -60,6 +60,13 @@ Pool.prototype.acquire = function(callback, priority) {
 
 // Release a connection back to the connection pool.
 Pool.prototype.release = function(connection, callback) {
+  // avoid re-adding failed connections to the connection pool
+  if (connection && connection.__knex__disposed) {
+    return setTimeout(function() {
+      callback();
+    },1);
+  }
+
   if (this.genericPool) {
     this.genericPool.release(connection, callback);
   } else {