Revert of IndexedDB: Only reset prefetch caches for cursors in same transaction (https://codereview.chromium.org/128713006/)

Reason for revert:
See previous comment.

Original issue's description:
> IndexedDB: Only reset prefetch caches for cursors in same transaction
> 
> Correct prefetching is predicated on resetting the cache when other
> requests come in that could invalidate it (i.e. writes) or result in
> requests processing out of order (i.e. everything). When prefetching
> was originally implemented, the IDB implementation only handled one
> transaction at a time, so resetting was done with a big hammer - reset
> all of a front-end's caches when any other request is made. This was
> acknowledged as future work during code review.
> 
> The future is now! Only reset caches for cursors from the same
> transaction as the triggering request, which should be a performance
> win for concurrent transactions in the same front-end.
> 
> BUG=329950
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=246245

TBR=dgrogan@chromium.org,tsepez@chromium.org,cmp@chromium.org,jam@chromium.org,jsbell@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=329950

Review URL: https://codereview.chromium.org/144093010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246264 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246264