Document replica set behavior changes in version 2.7

This issue is to collect behavior changes worth noting in the release notes or which need special attention.

Release Notes:

• Can no longer load a config with an _id name different from what was specified by the "replSet" command line (or config file) option.
• Replica Set Config Restriction
  • Can no longer have nodes with votes > 1
    • This was deprecated and printed a warning in mongod version 2.6
  • Arbiters can no longer have 0 votes
  • Unrecognized fields in a config now produce an invalid config and an error, rather than the fields being ignored.
  • Can no longer have "getLastErrorDefaults: .. w:0" in a config
• Definition of w:'majority' write concern changed to mean majority of voting nodes.
• Removed the local.slaves collection. Information is in the 'repl' section of serverStatus – you must explicitly request via arg.
  • Note: The local.slaves collection was a mirror of the internal map of all the slaves' replication progress
• You cannot add a 2.6 (pre 2.6.6) member to a 2.8 replica set because initial sync may fail – specifically listDatabases may fail when the cloner runs on the 2.6 member talking to a 2.8 member since slaveOk is not set.
• We can no longer parse oplog entries without v:2 – such entries were created by MongoDB 2.2 and prior. A 2.8 secondary will abort if it encounters such oplog entries.
• Secondaries may abort if they replay an oplog from 2.4 or prior with an index build op that would normally fail on a 2.6 or later primary.
• Built-in webserver no longer has replication information.
• Adding a voting member to a two node set in which only one member votes will need force: true.
  • We need to receive positive responses from a majority of voting members in a new config to accept that config.

Special Attention:

• Initial sync
  • Builds indexes more efficiently for each collection
  • Now applies oplog entries in batches using threads, like normal application
• Invalid Replica Set Config (votes>1, arbiter-no-vote)
  • Will not load – fix on primary, then restart node
  • Will be rejected leaving nodes with old config, until fixed on primary