mon: implement a simpler sync

mon: implement a simpler sync

The previous sync implementation was highly stateful and very complex.
This made it very hard to understand and to debug, and there were bugs
still lurking in the timeout code (at least).

Replace it with something much simpler:

 - sync providers are almost stateless.  they keep an iterator, identified
   by a unique cookie, that times out in a simple way.
 - sync requesters sync from whomever they fancy.  namely anyone with newer
   committed paxos state.

There are a few extra fields that might allow sync continuation later, but
this is complex and not necessary at this point.

Signed-off-by: Sage Weil <sage@inktank.com>