Reduce the number of Http Agent instances

[gsoldevila]

Born from #134362, this issue aims at improving memory footprint and performance by reducing the number of elasticsearch-js's Client instances. I identified two things that can be improved:

• Storing and reusing instances of the same type, created using core elasticsearch service's createClient(...). If the method is called more than once with the same type and configuration, providing the same instance for subsequent calls would have two benefits:
  • Avoiding creating a new ClusterClient instance at each call. Note that ClusterClient creates: 2 es-js Client instances + 2*nbrOfNodes pools + 2*nbrOfNodes connections + 2*nbrOfNodes Http Agents.
  • Reusing the existing ClusterClient's resources (connection pools and agents, with potential idle sockets ready to dispatch requests).
• Improving createClient(...)'s logic (or update the contract), so that the 2 ClusterClient instances (root vs scoped) are not created systematically. There are plugins that call this method and only use one of the two instances.

We find a good example of both in interactive-setup plugin: It exposes an elasticsearch service abstraction whose methods systematically createClient(...) and end up using only one of the 2 ES Client instances.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)