[cfile] Improve the hash function of pair<DataType, EncodingType>
[cfile] Improve the hash function of pair<DataType, EncodingType>

In the original implementation, because `DataType` part and
`EncodingType` part will use the same bits, it is easy to
have hash collisions.

For example, the hash values of <UINT32=4, BIT_SHUFFLE=6>
and <INT32=5, PLAIN_ENCODING=1> are both equal to `100101`

This change also removed an unused member from `TypeEncodingTraits`

Change-Id: Id141c51147ae674b9bee3016026a0b91cb76b5aa
Reviewed-on: http://gerrit.cloudera.org:8080/14644
Tested-by: Kudu Jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu