[Execution Node] Reduce memory used for ledger.Payload by replacing decoded Key with encoded key buffer

[fxamacker]

Problem

ledger.Payload can use less memory. Payload.Key requires 4 allocations for any key with 3 parts.

We don't use Payload.Key except in migration and reports so it's expensive to needlessly keep it in memory.

Also, Payload.Key is needlessly decoded while rebuilding mtrie during EN restart as well as checkpointing.

Updates #1744 #2585

The Proposed Solution

Replace decoded Payload.Key with encoded key buffer to reduce number of allocations and avoid needlessly decoding it during EN restarts and checkpointing.

[fxamacker]

Data held in RAM can be reduced by 25GB and allocations can be reduced by 803+ millions by keeping encoded Payload.Key in RAM instead of as decoded Ledger.Key:

• total decoded size (as ledger.Key): ~35GB
• total encoded size (as []byte): ~10GB

This is from mainnet17 checkpoint file (June 3).

[fxamacker]

PR #2585 further reduces memory by removing legacy controller from Ledger.Key. It reduces encoded key size by having less data to encode in Key.