Turn on entry cache

[gengliqi]

Always turn on the entry cache to make the raft entry be got even when the write batch has not been written to raft db yet.

Because of these 3 reasons below, we can ensure the entry can be got after handling the raft ready

1. the entries which have been written to raft db can be considered as persisted(ensured by the logic of async io)
2. the entries which have been persisted can be applied(ensured by raft-rs)
3. the entries which have been applied in entry cache can be removed(ensured by the logic of raft log gc)