implement committed entries pagination

[hicqu]

Signed-off-by: qupeng [email protected]

This is a re-implement for #356, except such things:

• the new configuration max_committed_size_per_ready isn't pushed into RaftLog
• new test cases are simplified into one, and it covers Ready and LightReady

[BusyJay]

Why not set it by Config?

[hicqu]

Why not set it by Config?

Using an Options makes it configurable. For example, we can set it to 8M generally, however if there is no available memory, we can set it to 0 to fetch nothing temporarely.

[BusyJay]

How about adding an API to mutate the configuration instead of adding a new parameter?

[BusyJay]

I recall there is already a PR implementing the feature #356, can you update the PR and pick to master?

[BusyJay]

test_commit_pagination_after_restart should also be ported.