Rework DV aggregation selection proofs

[KaloyanTanev]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

After the previous fix from this PR, another issue was spotted.

At each first slot of the epoch, aggregations from Prysm VC failed, as there were 2 go routines working simultanuously:

1. Checking if attester is an aggregator
2. Asking DV for selection proofs for an attester

The first go routine requires a selection proof by itself, however, it does not await on the second one. This resulted in the following errors:

vc0-prysm-1  | time="2025-04-01 15:34:18" level=error msg="Could not check if validator 0x86d8c7ec1efb is an aggregator" error="selection proof not found for the given slot=704 and validator_index=14" prefix=client

With this PR a couple of things are addressed, mainly to fix this issue, but also improve the load Prysm VC puts on a DV:

1. Await for selection proofs for DV to be written to disk, before checking for if DV attester is an aggregator.
2. Check for aggregation selection proof for DV only after 2/3 of the slot has passed.
3. Do not send selection proof requests for the next epoch. This puts unnecessary load of duplicate entries on the DV.

Which issues(s) does this PR fix?

N/A

Other notes for review

N/A