[Curp] Merge heartbeat logic with append entries

[markcty]

Currently, heartbeat and append entries are separated -- heartbeat will be triggered every 150ms. However, when the server is busy, there will be many append entries triggered in between two heartbeats. These append entries alone can keep the follower alive. Therefore, these heartbeats can be optimized out.

Originally posted by @rogercloud in https://github.com/datenlord/Xline/pull/85#discussion_r1034530768