mon: Paxos: finish queued proposals instead of clearing the list

By finishing these Contexts, we make sure the Contexts they enclose (to be
called once the proposal goes through) will behave as their were initially
planned:  for instance, a C_Command() may retry the command if a -EAGAIN
is passed to 'finish_contexts', while a C_Trimmed() will simply set
'going_to_trim' to false.

This aims at fixing at least a bug in which Paxos will stop trimming if an
election is triggered while a trim is queued but not yet finished.  Such
happens because it is the C_Trimmed() context that is responsible for
resetting 'going_to_trim' back to false.  By clearing all the contexts on
the proposal list instead of finishing them, we stay forever unable to
trim Paxos again as 'going_to_trim' will stay True till the end of time as
we know it.

Fixes: #4895

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