Add the support of the namespace replication

[git-hulk]

This PR implements the namespace replication between master and slaves, the detailed design is inside #1720. We added a new configuration repl-namespace-enabled to turn on/off this feature and it's disabled by default.

• If repl-namespace-enabled is set to no, it behaves the same as the previous. All namespace configurations are stored in the configuration file.

• if repl-namespace-enabled is set to yes, all namespace configurations will be removed from the configuration file, stored in DB, and then propagated to slaves. If the namespace has already existed in the slave, it will be overwritten by the master. Another significant change is that slaves cannot change namespaces if the replication is enabled.

This closes #1720

[PragmaTwice]

It just come to me that if we firstly set repl-namespace-enabled to yes, use kvrocks in a period of time, and then change it to no, will it process correctly? e.g. will these tokens in the storage be removed?

[git-hulk]

It just come to me that if we firstly set repl-namespace-enabled to yes, use kvrocks in a period of time, and then change it to no, will it process correctly? e.g. will these tokens in the storage be removed?

Good point, we should remove the DB key if this option switches from yes to no. But this may introduce another issue if the slave enables this configuration. So I think we can disallow changing it to no after turning it to yes.