Rather than performing a linear search for a validator by its address,
construct an index for the validator set sorted by address.  While
building the index takes O(N log N) time, it cuts lookup to O(log n).

On Solana, with ~50 validators, this change causes reduction of the
cost of a single lookup from 20k compute units to 700.

Furthermore, merge the index with the seen_validators map so that only
a single lookup is now performed which allows us to find the validator
and check whether it’s not a duplicate.

Co-authored-by: Dhruv D Jain <dhruv@iamsizzling.com>