[SSV] Fix cache behaviour when requesting blocks in parallel

[StefanBratanov]

There are scenarios where multiple operators may be connected to the same Beacon node,each of which requests the Beacon block proposal, in case they become the leader and must propose it on SSV. We have some cases in code, which cache data by slot for block proposal (with the usual assumption only one block proposal is requested per slot) so in case of two responses (one builder response and one fallback), there could be issues. On first look, the only two cases are in ExecutionLayerBlockProductionManagerImpl and in FailoverValidatorApiHandler.

More background on Discord and an issue occuring: https://discord.com/channels/697535391594446898/697539289042649190/1286291762167545958