ProtocolKeyRegistration creates performance bottleneck in aggregator

[jpraynaud]

Issue

During our stress test benchmarks of the aggregator, we have noticed that a bottleneck probably exists with the ProtocolKeyRegistration which is done for each new signature sent to the aggregator. Its creation is resource intensive and leads to quadratic computations. This could be easily fixed by memoizing the ProtocolKeyRegistration which is the same for all the signatures during an epoch

To do

• Add a service to hold a precomputed ProtocolMultiSigner (or SignerBuilder) for the current epoch
• Use this cached ProtocolMultiSigner in the multi signer functions verify_single_signature and create_multi_signature
• Verify that the delay to ingest delay for signatures of 500 signers in the aggregator has significantly dropped