mon/Paxos: fix pn for uncommitted value during collect/last phase

During the collect/last exchange, peers share any uncommitted values
with the leader.  They are supposed to also share the pn under which
that value was accepted, but were instead using the just-accepted pn
value.  This effectively meant that we *always* took the uncommitted
value; if there were multiples, which one we accepted depended on what
order the LAST messages arrived, not which pn the values were generated
under.

The specific failure sequence I observed:

 - collect
  - learned uncommitted value for 262 from myself
  - send collect with pn 901
 - got last with pn 901 (incorrect) for 200 (old) from peer
  - discard our own value, remember the other
 - finish collect phase
  - ignore old uncommitted value

Fix this by storing a pending_v and pending_pn value whenever we accept
a value.  Use this to send an appropriate pn value in the LAST reply
so that the leader can make it's decision about which uncommitted value
to accept based on accurate information.  Also use it when we learn
the uncommitted value from ourselves.

We could probably be more clever about storing less information here,
for example by omitting pending_v and clearing pending_pn at the
appropriate point, but that would be more fragile.  Similarly, we could
store a pn for *every* commit if we wanted to lay some groundwork for
having multiple uncommitted proposals in flight, but I don't want to
speculate about what is necessary or sufficient for a correct solution
there.

Fixes: #5698
Backport: cuttlefish, bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
mon/Paxos: add failure injection points

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