mon/Paxos: fix another uncommitted value corner case

It is possible that we begin the paxos recovery with an uncommitted
value for, say, commit 100.  During last/collect we discover 100 has been
committed already.  But also, another node provides an uncommitted value
for 101 with the same pn.  Currently, we refuse to learn it, because the
pn is not strictly > than our current uncommitted pn... even though it is
the next last_committed+1 value that we need.

There are two possible fixes here:

 - make this a >= as we can accept newer values from the same pn.
 - discard our uncommitted value metadata when we commit the value.

Let's do both!

Fixes: #6090
Signed-off-by: Sage Weil <sage@inktank.com>