Try to apply all committed entries in raft_periodic().

[yossigo]

I have noticed that entries accumulate in the log and only get applied in the rate of calling raft_periodic(). This greatly improves write rate, and as far as I can tell should not break anything.

Perhaps a future improvement can add a cap on the number of entries (or even time) per periodic call to prevent starvation of other parts of the event loop.

[willemt]

Applying one raft_apply_entry is just wrong. It's bad because:

• We usually want to apply many entries (not one)
• Applying multiple entries at a time is more efficient (ie. batching)

I will merge this in because of those reasons.

The one thing I am unsure about is:

• Would a user want to apply less than all the unapplied entries?

[yossigo]

I can see why a user would want to control the rate of applying entries. However, in this case I have a feeling the proper way to allow that would be to extend the apply callback so users can indicate the call was rejected and should not be retried for some time (i.e. library needs to bail out of the apply loop).