Provide API for obtaining all the WAL files

Currently RegionServerServices has the following API:

  WAL getWAL(HRegionInfo regionInfo) throws IOException;

Caller can only obtain filenum for a specific WAL.

When multi wal is in use, we should add API for obtaining highest file number among all the outstanding WAL files.

User can pass null to getWAL() method above, but the filenum for the returned WAL may not be the highest among all the WAL files.
See log snippet in the first comment.