mon: make paxos_size() unsigned

mon: make paxos_size() unsigned

* make paxos_size() unsigned, as paxos_size() returns the size of
  MonMap::mon_info, so it should be always a non-negative value,
  and more importantly, it represents a size.
* change the type of MonMap::removed_ranks from std::set<int>
  to std::set<unsigned>. for two reasons:
  - removed_ranks only tracks the rank which is greater or equal to 0
  - helps to silence the warnings listed below.
  MonMap::removed_ranks is persisted using encode()/decode(), but this
  change is backward compatible, as we use the raw encoder to encode
  signed and unsigned integers, the difference between the encoding
  schema between them only matters when MSB in the number is used,
  but this is not likely happen, as we neither have a negative
  rank in removed_ranks, no have a rank greater than `(unsigned)-1`,
  i.e., 0xffffffff.

this change partially reverts f75dfbc055ccf4e43b817ed5aa52898ff680e19e

to address the compiling warnings like:

/home/kefu/dev/ceph/src/mon/ElectionLogic.cc: In member function ‘void ElectionLogic::end_election_period()’:
/home/kefu/dev/ceph/src/mon/ElectionLogic.cc:173:23: error: comparison of integer expressions of different signedness: ‘std::set<int>::size_type’ {aka ‘long unsigned int’} and ‘int’ [-Werror=sign-compare]
  173 |       acked_me.size() > (elector->paxos_size() / 2)) {
      |       ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/kefu/dev/ceph/src/mon/ElectionLogic.cc: In member function ‘void ElectionLogic::propose_connectivity_handler(int, epoch_t, const ConnectionTracker*)’:
/home/kefu/dev/ceph/src/mon/ElectionLogic.cc:338:28: error: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Werror=sign-compare]
  338 |     for (unsigned i = 0; i < elector->paxos_size(); ++i) {
      |                          ~~^~~~~~~~~~~~~~~~~~~~~~~
/home/kefu/dev/ceph/src/mon/ElectionLogic.cc: In member function ‘void ElectionLogic::receive_ack(int, epoch_t)’:
/home/kefu/dev/ceph/src/mon/ElectionLogic.cc:469:25: error: comparison of integer expressions of different signedness: ‘std::set<int>::size_type’ {aka ‘long unsigned int’} and ‘int’ [-Werror=sign-compare]
  469 |     if (acked_me.size() == elector->paxos_size()) {
      |         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make[3]: *** [src/mon/CMakeFiles/mon.dir/build.make:328: src/mon/CMakeFiles/mon.dir/ElectionLogic.cc.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[3]: Leaving directory '/home/kefu/dev/ceph/build'
[ 48%] Built target libglobal_objs
/home/kefu/dev/ceph/src/mon/Elector.cc: In member function ‘void Elector::notify_rank_removed(int)’:
/home/kefu/dev/ceph/src/mon/Elector.cc:734:43: error: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Werror=sign-compare]
  734 |     for (unsigned i = rank_removed + 1; i <= paxos_size() ; ++i) {
      |                                         ~~^~~~~~~~~~~~~~~

Fixes: https://tracker.ceph.com/issues/56581
Signed-off-by: Kefu Chai <tchaikov@gmail.com>