raft: introduce log storage interface.

This change splits the raftLog.entries array into an in-memory
"unstable" list and a pluggable interface for retrieving entries that
have been persisted to disk. An in-memory implementation of this
interface is provided which behaves the same as the old version;
in a future commit etcdserver could replace the MemoryStorage with
one backed by the WAL.

test

@@ -39,7 +39,7 @@ split=(${TEST// / })
 TEST=${split[@]/#/${REPO_PATH}/}
 
 echo "Running tests..."
-go test ${COVER} $@ ${TEST} --race
+go test -timeout 10s ${COVER} $@ ${TEST} --race
 
 echo "Checking gofmt..."
 fmtRes=$(gofmt -l $FMT)