When a lot of transactions are occurring on the network, such as during
an NFT drop, it drives gas fees up. When this happens, we want to not
only inform the user about this, but also dissuade them from using a
higher gas fee (as we have proved in testing that high gas fees can
cause bidding wars and exacerbate the situation).
The method for determining whether the network is "busy" is already
handled by GasFeeController, which exposes a `networkCongestion`
property within the gas fee estimate data. If this number exceeds 0.66 —
meaning that the current base fee is above the 66th percentile among the
base fees over the last several days — then we determine that the
network is "busy".
The `SET_CURRENT_FIAT` action has been removed. It has been replaced
by a call to `forceUpdateMetamaskState`. The only purpose of this
action was to eagerly update the current fiat currency settings before
the next state update. Forcing a state update instead is simpler and
safer.
The `setCurrentCurrency` function in the background has been updated to
no longer return the state, now that it's no longer needed.
