SERVER-15570 Avoid querying system.indexes in ttl.cpp.

Changed so that the TTL indexes are processed in multiple phases:
  - Acquire an IS-mode lock on the database and find all of the
    TTL indexes

  For each TTL index,
  - Acquire an IX-mode lock on the collection and delete all of the
    expired documents

Also avoid checking `userFlags` of collection stats in TTL index
tests since it is not set by some storage engines.