rkv misses revision 2

[h-w-chen]

when inserting key-value to rkv, revison 2 is missed in the sequence

How to reproduce

start rkv server,
run following client commands to insert 3 k-v pairs:

$ curl -X POST 127.0.0.1:8090/kv -d '{"key":"testk", "value":"testv1"}'
The key value pair (testk,testv1) has been saved as revision 1 at 127.0.0.1:16378,127.0.0.1:16378,127.0.0.1:16378
$ curl -X POST 127.0.0.1:8090/kv -d '{"key":"testk", "value":"testv2"}'
The key value pair (testk,testv2) has been saved as revision 3 at 127.0.0.1:16378,127.0.0.1:16378,127.0.0.1:16378
$ curl -X POST 127.0.0.1:8090/kv -d '{"key":"testk", "value":"testv3"}'
The key value pair (testk,testv3) has been saved as revision 4 at 127.0.0.1:16378,127.0.0.1:16378,127.0.0.1:16378

notice that revisions return are 1, 3, 4.