If the initial network when the extension is started is something other
than Mainnet, the swaps controller will never successfully retrieve
swap quotes. This is because the `ethers` provider used by the swaps
controller doesn't allow network changes by default - it assumes that
the network remains the same as when the provider was initialized.
This was fixed by hard-coding Mainnet as the initial chain ID for this
`ethers` provider used by the swaps controller.
Some adjustments needed to be made to the `provider` stub to allow
setting `1` as the network ID and chain ID in unit tests.
* Calculate savings per swap relative to median values
* Update test mock quotes, add getMedian tests
* Identify assets by sourceToken and destinationToken
* Stop passing a gas param to the estimateGas call initiated in the swaps controller timedoutGasReturn
* Stop passing gas params to timedoutGasReturn
* Lint fix
* Stop passing no longer used param to setInitialGasEstimate
The `_fetchAndSetSwapsLiveness` was accidentally passed to
`setInterval` without being bound first, so the `this` reference was
not defined when it was called. It is now bound before being passed to
`setInterval`.
