[xfrout] (big) refactoring: use BUNDYServer to eliminate intermediate thread.

It also uses the generic socketsession module to get xfr queries from
bundy-auth.  this will eliminate the need for the in-house and yet incomplete
support of query forwarding.

a new tentative class NewXfroutServer is introduced to add the implementation
and unit tests until the auth side is updated (it's currently only used for
tests).  

Although the size of the diff is large, this is mostly a copy + adjustment of the original xfrout and
UnixSockServer classes, and of bundy-memmgr (which uses BUNDYServer), so this should be
a relatively low-risk change.  I hope someone can take a look at it for sanity check.  Otherwise
I'll merge it at my own discretion some time later.