Transaction Manager: configurable increaseGasPrice behavior

[axelKingsley]

Previously, the gas price in the transaction manager would use the generated estimate always, in a function called increaseGasPrices.

In this PR , it's being modified such that if the incoming transaction is already priced higher than the estimate, the higher value is always used.

This behavior is probably fine, but there may be customers who prefer (or even rely on) the limit going back down.

We should add into the configuration for SimpleTransactionManager, some way to specify the behavior, likely a simple flag.