Routing peering control plane traffic through mesh gateways can be
enabled or disabled at runtime with the mesh config entry.
This commit updates proxycfg to add or cancel watches for local servers
depending on this central config.
Note that WAN federation over mesh gateways is determined by a service
metadata flag, and any updates to the gateway service registration will
force the creation of a new snapshot. If enabled, WAN-fed over mesh
gateways will trigger a local server watch on initialize().
Because of this we will only add/remove server watches if WAN federation
over mesh gateways is disabled.
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