Optimize undefined gas limit for individual Beacon updates

[bbenligiray]

Blocked by api3dao/airseeker#447 and api3dao/airseeker#446

With self-funded Beacons, we also want to omit fulfillmentGasLimit (same reasons as api3dao/airseeker#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 api3dao/airseeker#446

[bbenligiray]

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