migrated multi host connection pool from godropbox package

removing unneeded dependencies, which involved etcd versions.

weed/wdclient/net2/doc.go

@@ -0,0 +1,6 @@
+// net2 is a collection of functions meant to supplement the capabilities
+// provided by the standard "net" package.
+package net2
+
+// copied from https://github.com/dropbox/godropbox/tree/master/net2
+// removed other dependencies