SERVER-5826 prevent building an index with a non-existent plugin
Missing write intent in doDBUpgrade()
SERVER-8751 Bump pdfile minor version on new index plugin creation

Changes:
1) Warn on startup if using old minor version and invalid indexes exist.
2) Treat unknown index plugins created under pre-2.4 as ascending
3) When creating first index using new plugins:
    a) if there are invalid indexes error out
    b) if not pump the pdfile minor version
4) ScanAndOrder no longer supports index plugins (it never should have)

Note:
All text and 2dsphere indexes created prior to this commit will need to be rebuilt.