Cache sanitized label names and return them next time.
This reduces the number of allocations and speeds up the SanitizeLabelName()
function for common case when the number of unique label names is smaller than 100k
* adds consul watch api,
it must reduce load on consul service with blocking wait requests,
changed discoveryClient api with fetchResponseMeta callback.
* small fix
* fix after master merge
* adds watch client at discovery utils
* fixes consul watcher,
changes namings,
fixes data race
* small typo fix
* sanity fix
* fix naming and service node update
