refactor: Refactor the method 'check_leader' make it return a enum class type

[WHBANG]

What problem does this PR solve?

#1405

The purpose of this patch is to make method check_leader return enumeration class
type(meta_leader_state) instead of int, to avoid making mistake.

Refactor the following two methods and the methods that use them：

int check_leader(dsn::message_ex *req, dsn::rpc_address *forward_address);

template <typename TRpcHolder>
int check_leader(TRpcHolder rpc, /*out*/ rpc_address *forward_address);

void meta_service::on_start_recovery(configuration_recovery_rpc rpc);

bool meta_service::check_status(TRpcHolder rpc, rpc_address *forward_address);