paxos: fix sharing of learned commits during collect/last

paxos: fix sharing of learned commits during collect/last

We can learn either an uncommitted or committed value during the
collect/last recovery phase.  For the committed values, we need to remember
each peer's first/last_committed and share only at the end to avoid a
situation like:

 - mon.1 has same last_committed as us
 - mon.2 has newer last_commited, we save it
 - mon.3 has same last_commited as mon.1, we share new value
 - done... but mon.1 never got mon.2's newer commit.

Instead, save the commit sharing until the collect process completes, so
we know that any committed value learned from anyone is shared with
everyone who needs it.

This fixes a crash like

mon/Paxos.cc: In function 'void Paxos::handle_begin(MMonPaxos*)', in thread '7fd91192c700'
mon/Paxos.cc: 400: FAILED assert(begin->last_committed == last_committed)
 ceph version 0.38-208-g9aabd39 (commit:9aabd3982cceb7e8489412b4bfbb4c2387880de2)
 1: (ceph::__ceph_assert_fail(char const*, char const*, int, char const*)+0x76) [0x72454e]
 2: (Paxos::handle_begin(MMonPaxos*)+0x363) [0x6499ef]
 3: (Paxos::dispatch(PaxosServiceMessage*)+0x2b4) [0x64db2c]
 4: (Monitor::_ms_dispatch(Message*)+0xdc6) [0x6205c2]
 5: (Monitor::ms_dispatch(Message*)+0x3a) [0x62831a]
 6: (Messenger::ms_deliver_dispatch(Message*)+0x63) [0x7d1f31]
 7: (SimpleMessenger::dispatch_entry()+0x7c2) [0x7bb786]
 8: (SimpleMessenger::DispatchThread::entry()+0x2c) [0x6070fa]
 9: (Thread::_entry_func(void*)+0x23) [0x6f3f69]
 10: (()+0x7971) [0x7fd9153a1971]
 11: (clone()+0x6d) [0x7fd913c3092d]

Signed-off-by: Sage Weil <sage@newdream.net>