plugin/transfer: Zone transfer plugin (#3223)

* transfer plugin

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/transfer/README.md

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+# transfer
+
+## Name
+
+*transfer* - answer zone transfers requests for compatible authoritative
+plugins.
+
+## Description
+
+This plugin answers zone transfers for authoritative plugins that implement
+`transfer.Transferer`.
+
+Transfer answers AXFR requests and IXFR requests with AXFR fallback if the
+zone has changed.
+
+Notifies are not currently supported.
+
+## Syntax
+
+~~~
+transfer [ZONE...] {
+  to HOST...
+}
+~~~
+
+* **ZONES** The zones *transfer* will answer zone requests for. If left blank,
+  the zones are inherited from the enclosing server block. To answer zone
+  transfers for a given zone, there must be another plugin in the same server
+  block that serves the same zone, and implements `transfer.Transferer`.
+
+* `to ` **HOST...** The hosts *transfer* will transfer to. Use `*` to permit
+  transfers to all hosts.