[WIP] *: configure server keepalive, optimize client balancer with gray-list #8463
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keepalive timed-out is 'connectivity.TransientFailure'
in gRPC; it keeps retrying (calling 'Balancer.Up') until
success. This is problematic in multi-endpoint balancer
with an endpoint being blackholed. Balancer can get stuck
retrying blackholed endpoint, taking several seconds to
find healthy ones.
Select only healthy endpoints on endpoints-notify.
Gray-list unhealthy endpoints, reverting after dial timeouts.
Update: closed in favor of #8545.