slack-15.0: support consul topo stale reads

[timvaillancourt]

Description

This PR allows Vitess components to use stale reads from consul. This is enabled with the flag --topo_allow_stale_reads, which is default false and should be considered experimental

*api.QueryOptions:

// AllowStale allows any Consul server (non-leader) to service
// a read. This allows for lower latency and higher throughput
AllowStale bool

This flag allow us to distribute topo .Get(...) and .List(...) load to consul replicas. Also it limits the impact of elections

It may be safe in use cases like txthrottler if stale reads have no impact to any other vttablet operations. It may be safe to use in vtorc also

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[ejortegau]

LGTM