Get rid of the AbtractWAL

Summary:
With the recent refactoring of the WAL we need to get rid of the
AbstractWAL and use the WALProvider.

This diff will just remove the use of AbstractWAL

Test Plan: hbase-consensus unit tests

Reviewers: reddragon, fantasist, eclark, tedyu

Differential Revision: https://reviews.facebook.net/D30681

Signed-off-by: Elliott Clark <eclark@apache.org>