Remove camel_store_summary_index().

No longer used.
Lock the CamelStoreSummary before invoking store_info_set_string().

Not the best approach since we're invoking unknown code with a mutex
locked, but beats the hell out of exposing the mutex in the public API.
Remove CAMEL_STORE_SUMMARY_SUMMARY_LOCK.

Keep this internal to CamelStoreSummary.

Removed functions:

  camel_store_summary_lock()
  camel_store_summary_unlock()