HAMMER VFS - Refactor merged search function to try to avoid missed entries

HAMMER VFS - Refactor merged search function to try to avoid missed entries

Refactor the merged B-Tree + In-Memory search function to try to avoid races
where an in-memory record is flushed to the media during a search, causing
the search to miss the record.

Add another flag to hammer_record_t to indicate that the record was deleted
because it was committed to the media (verses simply being deleted).

Better-separate HAMMER_RECF_DELETED_FE and HAMMER_RECF_DELETED_BE.  These
flags indicate whether the frontend or backend deleted an in-memory record.
The backend ignores frontend deletions that occur after the record has been
associated with a flush group.

Remove some console Warnings that are no longer applicable.