Add batch request support for transport of cluster flow control

[sczyh30]

Issue Description

Type: feature request

Describe what happened (or what feature you want)

In the current implementation, if there are multiple cluster rules (=N) of a resource, when checking for cluster rules, the token client will send up to N requests to the token server respectively to acquire tokens. Thus the performance might be bad when there're many cluster rules (e.g. gateway rules for different types of HTTP request items) and the network is not well. We might need to add batch request support for TokenService and cluster transport implementations to handle this.

[linlinisme]

give it to me :)

[sczyh30]

give it to me :)

Thanks! But we've already finished an initial version for this. Maybe you could help us to review the code when the PR is ready :)

[linlinisme]

ok~

[sczyh30]

@linlinisme You could refer to #850 for review.