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

Currently shows no significant quality
degredation or speed improvement, and set to off
by default. Requires hash_me, lv_map and
lv_map_multi. Adding to speed features required
changing AV1_COMMON *cm to AV1_COMP *cpi in a
chain of functions.

Variations that have been tried:
-varying the maximum eob on which the feature
activates: 16, 32, 64. 16 currently used. 64
has best hit rate but longer execution time.
-varying the data hashed and the length of hashes
(first hash is 16 bit and based on context data,
while second hash is 16 bit and based only on
pre-optimized qcoeff values.)
-softening the data used for the hashes: ideally
this would raise the number of hits, without
compromising quality too much.

Change-Id: I94f22be82f3a46637c0489d512f2e334a307575f