Audit the use of Array.slice()

[twoeths]

Is your feature request related to a problem? Please describe.

Some Array.slice() is used unnecessarily just for api convenience, this not only affects performance (as a new array is copied) but also cause gc to run more frequently. Some examples:

1. archiveBlocks.ts:

for (let i = 0; i < blocks.length; i += BATCH_SIZE) {
    const toIdx = Math.min(i + BATCH_SIZE, blocks.length);
    const canonicalBlocks = blocks.slice(i, toIdx);

then we loop canonicalBlocks. In this case we can just do a for loop over i and toIdx

2. stateContextCache

prune(headStateRootHex: RootHex): void {
    const keys = Array.from(this.cache.keys());
    if (keys.length > this.maxStates) {
      // object keys are stored in insertion order, delete keys starting from the front
      for (const key of keys.slice(0, keys.length - this.maxStates)) {

in this case we can just do a regular for loop and break if it goes beyond the limit.

Describe the solution you'd like

Audit the use of Array.slice() in lodestar, only use it if it's really needed