Add speed feature that uses hash tables to
reuse deltas from previously found optimized
coefficients in av1_optimize_txb. This skips some
expensive optimize_txb calls.

Work stopped (no current benefit):
Current setup has worse speed (~1%), and is
turned off by default in speed_features.c.
Overall PSNR was ~0.1% worse. See
go/hashBasedTrellis for feature details.

Requires hash_me.

Change-Id: Id12ada82984ea6682e2b6f27534fa57f3a3abb4a