Add State Read Ahead Stage

[mh0lt]

The BAL contains the hash keays read for the block this cah be read ahead of execution to warm the state cache.

It likely has the following requirements:

1. It should use the same shared domain as execution so the reads are locally cached (This assumes that the shared domain object has been upgraded to hold a get cache as well as a put cache).
2. It can run in parallel - i.e. the stage can complete once it has started reading so updates are paralelized
3. It should stop execiting if its not compete once execution has completed - we need to check behaviour there in practice.

We also need to test that this read ahead makes a difference to throughput otherwise it may not be worth running

[VBulikov]

mh0lt added Imp2 as Importance

[AskAlexSharov]

FYI: now we have

var readAhead chan uint64
	// snapshots are often stored on chaper drives. don't expect low-read-latency and manually read-ahead.
	// can't use OS-level ReadAhead - because Data >> RAM
	// it also warmsup state a bit - by touching senders/coninbase accounts and code
	if !execStage.CurrentSyncCycle.IsInitialCycle {
		var clean func()

		readAhead, clean = exec.BlocksReadAhead(ctx, 2, cfg.db, cfg.engine, cfg.blockReader)
		defer clean()
	}

[VBulikov]

yperbasis changed Importance from Imp2 to Imp1