add DAC requests a rate limit and if error avoid requests for a period of time

[joanestebanr]

Closes #95

Rationale

If a DAC Server is returing error, avoid to do request for a while

Implementation

You can configure the cool down period if error. It's not going to do new requests until RetryOnDACErrorInterval reach:

[Etherman.Validium]
		RetryOnDACErrorInterval = "5m"

Add a rate limit:

[Etherman.Validium.RateLimit]
			NumRequests = 900
			Interval = "1s"

To disable it set NumRequests to 0
It allows to do NumRequests in the interval Interval if try to do more requests it sleeps until fulfill the rate