Maintain data plane client connections concurrently

[levikobi]

#93 introduced support for maintaining multiple gRPC client connections. However, the creation and destruction of the connections is done serially. First, we're closing outdated connections and then creating new connections, each happens one by one.
In large clusters, this will introduce significant delays at the reconciliation phase of the data-plane controller.

• All new connections should be established concurrently.
• All old connections should be destroyed concurrently.
• We should create and destroy connections concurrently.

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/lifecycle stale

[shaneutt]

Not stale, wanted for https://github.com/kubernetes-sigs/blixt/milestone/6

/remove-lifecycle stale

[shaneutt]

planned for v0.6.0

/lifecycle frozen