Avoid increasing connection count to passive secondaries with matchPrimaryNode

the matchPrimaryNode mode for the sharding connection pool currently spools as many connections to priority 0 nodes as to electable nodes. This isn't desirable, because those nodes won't become primary (so we don't need to stock as many connections to them).

We have this information from isMaster (in the form of the "passive" and "passives" fields), so should expose it in the rsm + use it for matchPrimaryNode