This commit re-structures the speed feature setup for the codec
development purpose. Instead of progressively reducing encoder
complexity at the expense of incremental coding loss, we allow a
separate set of speed features, each corresponds to a certain
category of coding units:

1 << 0: transform coding
1 << 1: inter prediction
1 << 2: intra prediction
1 << 3: block partition
1 << 4: loop filters
1 << 5: rd early skip

[6 - 7] are left open for next adjustment.

It is constructed to facilitate the codec development purpose.
When working on a coding functions, one could choose to turn on
one or more less related coding units to speed up the evaluation
process. For example, to test a transform related experiment, one
could set
--dev-sf=2, 6, or 22
which corresponds to turning on:
2 - inter prediction speed features,
6 - both inter / intra speed features,
22 - inter / intra, and loop filter features.

The goal is to allow faster experimental verification during the
development process. With the experiment in a stable state, we
can evaluate its performance in speed 0 at higher confidence level.

Change-Id: Ib46c7dea2d2a60204c399dc01f10262c976adf0d