Optimize undefined gas limit for individual Beacon updates

[bbenligiray]

Blocked by #447 and #446

With self-funded Beacons, we also want to omit fulfillmentGasLimit (same reasons as #447), but we don't want to call estimateGas() for each update because it's wasteful (they will all return the same value). As a solution, in cases that fulfillmentGasLimit is not defined and the transaction to be made is a single Beacon update, we can use the estimateGas() result we got from #446

[bbenligiray]

@metobom Let's also drop this from Phase 2 to high priority Post-phase 2