Add pending tx checks for keeper / revealer accounts

[sohkai]

Given how volatile the gas market currently is, it would be ideal to:

1. Have alerts if there are pending transactions stuck in an account's tx queue for a while
2. Automatically detect and reprice these transactions, if they are time-sensitive

Given that we have some slack in terms of updating the heartbeat and completing the reveals / settlements, the automation may not be worth it at the moment.

In terms of detecting pending transactions, this is likely something the Etherscan API would be well suited for, as it has a better overview of the global tx mempool than just a single node or provider.

[promaty]

I will add a continuous etherscan query on every keeper run, expose that as prometheus gauge and add an alert, should be good.
Need to extend our prometheus library a bit, since we only support counters now (+-1 to existing number)

[promaty]

Added monitor for failing transactions in #216
However not sure how to proceed with pending transactions since Etherscan does not expose any api for these

[sohkai]

It's kind of roundabout, but one way we could detect this is to keep a record of the last transaction we've sent and alert if it doesn't get mined in X hours.