Exporter: validators order might be changed during a full sync

[amirylm]

Describe the bug

The issue is originated in the way the exporter adds an internal index to validators, which works async with events.Feed and depends on an external task (get validator data from beacon node), and therefore could result in different order when a large amount of validators are added simultaneously (as in full re-sync)

To Reproduce

Expected behavior

Screenshots

Node (please complete the following information):

• Operator Version: v0.1.9

Additional context