pm: Update tx cost check for faceValue()

[yondonfu]

What does this pull request do? Explain your changes. (required)

This PR addresses the problem described in #2049 which can occur when there are really big gas price spikes (think 500+ gwei) on mainnet that last for a relatively short period of time.

The solution in this PR contains an update to the solution presented in #2049. The tx cost check should not be completely removed because then it is possible for an O to do a bunch of transcoding without ever being able to redeem any tickets profitably. Instead, the tx cost check is updated such that O calculates the tx cost based on the current estimated gas price and the average estimated gas price (which could be calculated over a time period such as a few hours, a day, etc.). If there is a gas price spike and the desired ticket face value is less than the tx cost based on the current gas price then O also checks the face value against the tx cost based on the average gas price. If this fallback check passes then O proceeds to generate ticket params. The idea is that typically the current gas price will be less than or equal to the average gas price so the fallback check never runs, but if the current gas price exceeds the average gas price then as long as the average gas price check passes there is still a reasonable chance that the current gas price falls at some point such that the ticket face value can cover the tx cost based on the current gas price.

In this first pass, the average gas price value is hardcoded for now and the intent is to replace the hardcoded value with a dynamic way to determine the average gas price over a period of time later (i.e. using the ETH JSON-RPC feeHistory API or in a GasPriceMonitor implementation).

Specific updates (required)

See commit history.

How did you test each of these updates (required)

Updated unit tests.

Does this pull request close any open issues?

Fixes #2049

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[darkdarkdragon]

LGTM