SERVER-9516 Support schemaV24 users, for use prior to auth schema upgrade.

To enable this support, one must change the _version field when constructing the
AuthorizationManager, which is not possible in this change set.  A separate
change will introduce proper version detection and writing of
admin.system.version, at which point this functionality can be enabled.
SERVER-9516 Introduce symbolic constants for authz schema versions, renumber them.
SERVER-9516 Do not alter indexes on admin.system.users at startup unless using schemaVersion26Final.

Older schema versions may have incompatible documents in admin.system.users.
SERVER-9516 Write version document to admin.system.version as needed in mongod.

At startup, for standalone mongod and master.  Upon election to primary for replicaset members.