rpc/config: fix incorrect default value of wal_dir.
rpc/config: fix incorrect default value of wal_dir.

The default value of wal_dir was assumed to be "{name}.consensus", but was always "default.consensus", since FLAGS_name
was not set when the default value of wal_dir was determined.
Therefore we reset the default value of wal_dir in its validator.

neverchanje/yaraft-wal,neverchanje/consensus-yaraft,neverchanje/consensus-yaraft,neverchanje/consensus-yaraft,neverchanje/consensus-yaraft,neverchanje/yaraft-wal,neverchanje/yaraft-wal