Improve validator exit flow

[twoeths]

Motivation

• Processing a block with 16 validator exits is 100x times slower than regular block
• Slashing validator triggers validator exit also

Description

• Most of the time was for looping through validators, create a cache (ValidatorExitProcess) per block to make sure we loop through it at most 1 time
• Minimize for loop inside initiateValidatorExit
• The result in block.perf.ts is reduced from 100x to 6x, notice that processing 1 validator exit takes almost the same time to processing 16 validator exits

// Process block
// ================================================================
// Process block with 0 validator exit                                    233.6434 ops/s      4.280027 ms/op   3491 runs    15.01 s
// Process block with 1 validator exit                                    41.33581 ops/s      24.19210 ms/op    619 runs    15.00 s
// Process block with 16 validator exits                                  42.34492 ops/s      23.61558 ms/op    635 runs    15.02 s

Closes #2342

[codeclimate]

Code Climate has analyzed commit 842e889 and detected 8 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	2
Duplication	6

View more on Code Climate.

[dapplion]

Awesome performance boost! Tho I think it would be simpler to refactor the ValidatorExitProcess into a BlockProcess and make it mandatory param in the function signature.

[dapplion]

Looks great! Nice perf boost 🚀 We should add a benchmark for attester slashing and proposer slashing too