Fix AppendEntry validity checks to take the SnapshotIndex into account

This Jira aims to fix the following:

1. Before sending an appendEntry request to follower, leader checks the validity of the request be verifying if the follower has the previous log entry. But if the follower had installed a snapshot, the previous could be missing and the appendEntry would still be valid. Hence, the SnapshotIndex should be factored in while checking the validity of appendEntry request. Leader should store Follower's SnapshotIndex for this.
2. When follower receives appendEntry request, it checks the validity of log entry - the first index of the log entry is exactly 1 more than the last log index. During this check, the snapshotIndex should also be considered i.e. the first index of the log entry can be 1 more than the last log index or the snapshotIndex.
3. After Ratis server is restared, it loads all the available log segments. But logs with end index < last snapshot index should not be loaded. There can be gaps in the log segments upto the snapshotIndex if a snapshot was installed from leader node.