Dhcp-Ddns server design

[Thomas Markwalder]

Hello all:

The high level design for the Dhcp-Ddns Server (aka D2) on the wiki has 
been updated based on F2F review comments:

http://bind10.isc.org/wiki/DhcpDdnsDesign

Most notable changes are replacement of the "DdnsTransaction" class and 
its state model which handled a single DDNS update/response exchange, 
for NameAddTransaction and NameRemoveTransaction whose state models 
encompass the update processing as specified by RFC 4703.   A new class 
DnsClient will handle a single request-response exchange.

DnsServerMgr class has been removed entirely.