shardsManager: fix unchanged shard assignment causes exception

[mattisonchao]

Issue

#170

RCA

Oxia-java client didn't handle well with unchanged assignments. which calls the toMap method with the duplicated key and causes an exception throw in the GRPC stream.

The real exception capture by Arthas

java.lang.IllegalStateException: Duplicate key 36 (attempted merging values Shard[id=36, leader=***********, hashRange=HashRange[minInclusive=0, maxInclusive=4294967295]] and Shard[id=36, leader=***********, hashRange=HashRange[minInclusive=0, maxInclusive=4294967295]])

Why can the client receive notification about unchanged assignments?

1. The coordinator sends node-level assignments to the oxia server.
2. The oxia server will filter the namespace without an equals check and then send it to the client. (maybe caused by cost/data consistent consideration)

Modification

• Add merge function to avoid exception
• Print the root cause messages.