separate ConnectionPool from Cluster definition, make it sharable across Envoy worker threads

sometimes we deploy multiple services among a set of endpoints, we define a set of clusters on these machines, each has a pool connecting to the same set of endpoints.

Separating ConnectionPool from Cluster has many advantages:
* The same set of backends may be used to serve multiple services, they can share the same connections, better performance, less RAM footprint, less pool warming.
* different LB algorithms on same set of connections,
* possibly introduce context awareness between services/clusters when their backends are the same.