mon: Paxos: get rid of the 'prepare_bootstrap()' mechanism

mon: Paxos: get rid of the 'prepare_bootstrap()' mechanism

We don't need it after all.  If we are in the middle of some proposal,
then we guarantee that said proposal is likely to be retried.  If we
haven't yet proposed, then it's forever more likely that a client will
eventually retry the message that triggered this proposal.

Basically, this mechanism attempted at fixing a non-problem, and was in
fact triggering some unforeseen issues that would have required increasing
the code complexity for no good reason.

Fixes: #5102
Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>