crimson/osd: prevent premature OSD activation.

crimson/osd: prevent premature OSD activation.

In contrast to the classical OSD:

```
int OSD::init()
{
  // ...

  {
    epoch_t bind_epoch = osdmap->get_epoch();
    service.set_epochs(NULL, NULL, &bind_epoch);
  }

  // ...

  // load up pgs (as they previously existed)
  load_pgs();
```

crimson doesn't set the `bind_epoch` when initializing. The net
result is going active prematurely which happens because the 3rd
condition (`bind_epoch < osdmap->get_up_from(whoami)`) is always
true.

```
    if (osdmap->is_up(whoami) &&
        osdmap->get_addrs(whoami) == public_msgr->get_myaddrs() &&
        bind_epoch < osdmap->get_up_from(whoami)) {
      if (state.is_booting()) {
        logger().info("osd.{}: activating...", whoami);
```

Nullifying it translates the "is it activated?" check basically
into "is it up?" verification. This is problematic in a situation
like:

1. Primary got new OSDMap but replica has not.
2. Replica restarts, sends `MOSDBoot` and receives the newer map
   from the previous point.
3. Primary sends a message that is unexpected by replica.
4. Monitor publishes a new OSDMap diven by the `MOSDBoot`.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>