Improve work distribution among workers

[elopez]

Describe the desired feature

Currently Echidna does a very naive work distribution at the beginning of a campaign

echidna/lib/Echidna/UI.hs

Lines 87 to 90 in cde590f

	-- Distribute over all workers, could be slightly bigger overall due to
	-- ceiling but this doesn't matter
	perWorkerTestLimit = ceiling
	(fromIntegral conf.campaignConf.testLimit / fromIntegral nFuzzWorkers :: Double)

On long campaigns, this could result in several workers finishing significantly before the end of the campaign when they get to execute simpler, faster transactions, resulting in an overall reduction of execution speed towards the end of a campaign.