migrated multi host connection pool from godropbox package

removing unneeded dependencies, which involved etcd versions.

weed/wdclient/resource_pool/doc.go

@@ -0,0 +1,5 @@
+// A generic resource pool for managing resources such as network connections.
+package resource_pool
+
+// copied from https://github.com/dropbox/godropbox/tree/master/resource_pool
+// removed other dependencies